dt-bindings: net: micrel: Convert micrel-ksz90x1.txt to DT schema

dt-bindings: net: micrel: Convert micrel-ksz90x1.txt to DT schema

Convert the micrel-ksz90x1.txt to DT schema. Create a separate YAML file
for this PHY series. The old naming of ksz90x1 would be misleading in
this case, so rename it to gigabit, as it contains ksz9xx1 and lan8xxx
gigabit PHYs.

Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20260116130948.79558-3-eichest@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>