gpio: zynq: add missing module_exit function

This driver is tristate, so it should be cleanable.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>