dt-bindings: net: Document Marvell PHY "marvell,reg-init"

The Marvell PHY driver consumes marvell,reg-init as one or more
<page register mask value> tuples. Define the property so dtbs_check
validates the existing board descriptions.

Existing users have no compatible or just "ethernet-phy-ieee802.3-c22"
so this has to go into the generic ethernet-phy.yaml schema. Marking it
deprecated both for that reason and because opaque register
initialization values are discouraged in DT.

Assisted-by: LLM
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20260925140410.1157036-2-robh@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>