dt-bindings: net: dsa: mediatek,mt7530: Add missing 'reg' property

dt-bindings: net: dsa: mediatek,mt7530: Add missing 'reg' property

The 'reg' property is missing from the mediatek,mt7530 schema which
results in the following warning once 'unevaluatedProperties' is fixed:

Documentation/devicetree/bindings/net/dsa/mediatek,mt7530.example.dtb: switch@0: Unevaluated properties are not allowed ('reg' was unexpected)

Fixes: e0dda3119741 ("dt-bindings: net: dsa: convert binding for mediatek switches")
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20220701222240.1706272-1-robh@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>