net: dsa: ksz: Add optional reset GPIO to Microchip KSZ switch binding

Add optional reset GPIO, as such a signal is available on the KSZ switches.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Woojung Huh <Woojung.Huh@microchip.com>
Cc: David S. Miller <davem@davemloft.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>