i2c: mux: reg: use device property accessors

Convert the device-tree parsing path to the generic fwnode/device
property accessors so the driver can be probed on ACPI and swnode
platforms as well as OF. The helper is renamed from
i2c_mux_reg_probe_dt() to i2c_mux_reg_probe_fw() to reflect that.

Accessor translation:

  of_parse_phandle("i2c-parent") +
    of_find_i2c_adapter_by_node()    -> fwnode_find_reference() +
                                         i2c_find_adapter_by_fwnode()
  of_get_child_count()               -> device_get_child_node_count()
  of_property_read_bool()            -> device_property_read_bool()
  for_each_child_of_node()           -> device_for_each_child_node()
    of_property_read_u32("reg")        on ACPI device nodes:
                                         acpi_get_local_address()
                                       everything else (OF, swnode,
                                       ACPI data nodes):
                                         fwnode_property_read_u32()
  of_property_read_u32("idle-state") -> device_property_read_u32()

The child-node branch uses is_acpi_device_node() rather than
is_acpi_node(): the latter also matches ACPI data nodes (the
_DSD hierarchical-property children used by PRP0001-style
firmware), which have no ACPI handle and would make
acpi_get_local_address() fall back to evaluating _ADR against the
root namespace and return -ENODATA.  Routing data nodes through
fwnode_property_read_u32() instead lets them resolve the "reg"
property the same way OF and swnode children do.

Behavioural preservations (deliberate, to avoid regressing existing
users):

  - The three-way endian fallback is kept verbatim: an explicit
    "little-endian" property wins, then "big-endian", and otherwise
    the host's compile-time byte order. device_is_big_endian() is
    not used here because it ignores "little-endian" and introduces
    "native-endian" semantics, which would diverge from the binding.

  - The "if (!mux->data.reg)" guard around
    devm_platform_get_and_ioremap_resource() in probe() is kept.
    drivers/platform/mellanox/mlx-platform.c registers i2c-mux-reg
    platform_devices with no memory resource and supplies a
    pre-set .reg / .reg_size through struct
    i2c_mux_reg_platform_data; without the guard those
    registrations would fail in probe().

  - The "if (!mux->data.reg)" ioremap block (and the paired
    reg_size validation that depends on it) is hoisted above
    i2c_get_adapter(mux->data.parent), so the fwnode path
    preserves master's ordering of "ioremap before parent-adapter
    get".  For platdata users the validation runs from a slightly
    earlier position, but mux->data.reg_size is already set from
    platdata by then, so the order is functionally neutral.

The OF-only of_address_to_resource() translation in the old
probe_dt() is dropped because the same address is available from
the platform_device resource table on OF as well as ACPI, and the
existing fallback in probe() ioremaps it.

Acked-by: Peter Rosin <peda@lysator.liu.se>
Signed-off-by: Abdurrahman Hussain <abdurrahman@nexthop.ai>
Assisted-by: Claude-Code:claude-opus-4-7
Assisted-by: sashiko:gemini-3.1-pro-preview
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>

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