dt-bindings: net: dsa: mediatek,mt7530: expand gpio-controller description

Expand the description of the gpio-controller property to include the
controllable pins of the MT7530 switch.

The gpio-controller property is only used for the MT7530 switch. Therefore,
invalidate it for the MT7531 switch.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>