arm64: dts: apm: add required gpio-cells to DW APB GPIO controller port

The Synopsys DesignWare APB GPIO controller port must have gpio-cells
property, as pointed by dtschema:

  arch/arm64/boot/dts/apm/apm-mustang.dt.yaml: gpio@1c024000: gpio-controller@0: '#gpio-cells' is a required property

Link: https://lore.kernel.org/r/20200917165040.22908-2-krzk@kernel.org


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