phy: lynx-28g: support individual lanes as OF PHY providers

Currently, the bindings of this multi-lane SerDes are such that
consumers specify the lane index in the PHY cell, and the lane itself is
not described in the device tree.

It is desirable to describe individual Lynx 28G SerDes lanes in the
device tree, in order to be able to customize electrical properties such
as those in Documentation/devicetree/bindings/phy/transmit-amplitude.yaml
(or others).

If each lane may have an OF node, it appears natural for consumers to
have their "phys" phandle point to that OF node.

The problem is that transitioning between one format and another is a
breaking change. The bindings of the 28G Lynx SerDes can themselves be
extended in a backward-compatible way, but the consumers cannot be
modified without breaking them.

Namely, if we have:

&mac {
	phys = <&serdes1 0>;
};

we cannot update the device tree to:

&mac {
	phys = <&serdes1_lane_0>;
};

because old kernels cannot resolve this phandle to a valid PHY.

The proposal here is to keep tolerating existing device trees, which are
not supposed to be changed, but modify lynx_28g_xlate() to also resolve
the new format with #phy-cells = <0> in the lanes.

This way we support 3 modes:
- Legacy device trees, no OF nodes for lanes
- New device trees, OF nodes for lanes and "phys" phandle points towards
  them
- Hybrid device trees, OF nodes for lanes (to describe electrical
  parameters), but "phys" phandle points towards the SerDes top-level
  provider

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20251125114847.804961-4-vladimir.oltean@nxp.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

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