arm64: a37x0_gpio: Use syscon instead of MMIO region

The fdt node for this driver is a simple-mfd and syscon compatible one
meaning that simplemfd will be the driver attached for it. The gpio driver
is attached to the 'gpio' subnode so use syscon_get_handle_default to
obtain the handle of the syscon from the parent device and use this
to read/write to the memory region.

MFC after: 1 week