drivers: gpio: xlnx_ps: switch driver over to DEVICE_MMIO mapping

Set up a named device MMIO memory mapping in the GPIO controller's
parent device, map the virtual memory in the init function of the
parent device.

Once the controller's register space has been successfully mapped,
propagate the mapped virtual address to all child (= GPIO bank)
devices. While it is possible to add a named mapping to every
single GPIO bank device and initialize it in the respective bank
device's init function, this would result in multiple virtual
address mappings all pointing to the same 4k of physical memory.
I assume that, although all those mappings having the same attri-
butes, such a setup is at least discouraged.

Signed-off-by: Immo Birnbaum <mail@birnbaum.immo>