gpio: xilinx: Fix kernel-doc warnings

Fix kernel-doc format:
- Add gpio-width description
- Remove additional "inited" variable description
- Add return value description - xgpio_remove

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>