software node: add fw_devlink support

Software nodes can be used to describe supplier-consumer relationships
between devices they represent using reference property entries. Unlike
for OF-nodes, driver core cannot yet use these references to create a
probe order that avoids needless probe deferrals on missing providers.

Implement software_node_add_links() modelled on of_fwnode_add_links().
For every DEV_PROP_REF property we resolve each referenced supplier and
create an fwnode link from the node to it. The driver core later promotes
these to device links and defers the consumer until the suppliers are
ready.

There's no allowlist like the one DT needs - devicetree phandles appear
in plenty of non-supplier contexts, but a software node only carries a
reference property when its author explicitly points at another node, so
we treat every reference as an intentional supplier dependency and link
all of them. Graph "remote-endpoint" references are skipped for now: they
go 2-ways between endpoint nodes and would create graph cycles without
the port-parent lifting DT does via get_con_dev(). References to
suppliers that aren't registered yet and self-references are ignored.

fw_devlink resolves the supplier device through fwnode->dev but the core
only records the owning device on the primary fwnode. When the software
node is a device's secondary fwnode, mirror the device pointer onto it in
software_node_notify() so the consumer can actually find the supplier
instead of deferring forever.

While at it: purge the fwnode links in software_node_release() now that
software nodes can own them.

Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Link: https://patch.msgid.link/20260713-swnode-fw-devlink-v4-2-d4f2dee27ad9@oss.qualcomm.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

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