dt-bindings: net: cdns,macb: add a compatible for Microchip pic64hpsc

dt-bindings: net: cdns,macb: add a compatible for Microchip pic64hpsc

Add "microchip,pic64hpsc-gem" for "PIC64-HPSC" and
"microchip,pic64hx-gem" for "PIC64HX", compatible with the former.

The generic compatible "cdns,gem" works but offers limited features.
Keep it as a fallback.

The GEM IPs within pic64hpsc have their MDIO controllers unconnected
from any physical pin. Add a check to prevent adding PHYs under the GEM
node.

Signed-off-by: Charles Perry <charles.perry@microchip.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20260313140610.3681752-2-charles.perry@microchip.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>