dt-bindings: net: add STM32MP13 compatible in documentation for stm32

New STM32 SOC have 2 GMACs instances.
GMAC IP version is SNPS 4.20.

Signed-off-by: Christophe Roullier <christophe.roullier@foss.st.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>