dt-bindings: net: cdns,macb: document polarfire soc's macb

Until now the PolarFire SoC (MPFS) has been using the generic
"cdns,macb" compatible but has optional reset support. Add a specific
compatible which falls back to the currently used generic binding.

Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>