dt-bindings: net: dsa: microchip: add wakeup-source property

Add wakeup-source property to enable Wake on Lan functionality in the
switch.

Since PME wake pin is not always attached to the SoC, use wakeup-source
instead of wakeup-gpios

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>