arm64: dts: apm: drop unused reg-io-width from DW APB GPIO controller

The Synopsys DesignWare APB GPIO controller driver does not parse
reg-io-width and dtschema does not allow it so drop it to fix dtschema
warnings like:

  arch/arm64/boot/dts/apm/apm-mustang.dt.yaml: gpio@1c024000:
    'reg-io-width' does not match any of the regexes: '^gpio-(port|controller)@[0-9a-f]+$', 'pinctrl-[0-9]+'

Link: https://lore.kernel.org/r/20200917165040.22908-1-krzk@kernel.org


Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Olof Johansson <olof@lixom.net>