gpio/xilinx: Convert the driver to platform device interface

This way we do not need to transverse the device tree manually and we
support hot plugged devices.

Also Implement remove callback so the driver can be unloaded

Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>