arm64: dts: imx8mm-emcon-avari: Fix gpio-cells

Per nxp,pcf8575.yaml, #gpio-cells should be 2.

Change it to fix the following schema warning:

imx8mm-emcon-avari.dtb: gpio@3a: #gpio-cells:0:0: 2 was expected
	from schema $id: http://devicetree.org/schemas/gpio/nxp,pcf8575.yaml#



Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>