dt-bindings: net: dsa: microchip: add interrupts property for ksz switches

The ksz switch driver allows specifying an interrupt line to prevent
having to periodically poll the switch for link ups/downs and other
asynchronous events.

Signed-off-by: Leonard Göhrs <l.goehrs@pengutronix.de>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>