ARM: dts: lan966x: Add gpio-restart

The pcb8291 can be rebooted by toggling the GPIO 56. Therefore enable
this in DT.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220719201158.1696168-2-horatiu.vultur@microchip.com