gpio/xilinx: Fix kernel-doc

Some documentation were not following the kernel-doc format.
Backporting patch from Xilinx git repository.

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