gpio/xilinx: Add support for X86 Arch

Core can be accessed via PCIe on X86 platform.
This patch also allows the driver to be used as module.

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>