gpio: xilinx: remove excess kernel doc

gpio: xilinx: remove excess kernel doc

The irqchip field has been removed from struct xgpio_instance so remove
the doc as well.

Fixes: b4510f8fd5d0 ("gpio: xilinx: Convert to immutable irq_chip")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202312150239.IyuTVvrL-lkp@intel.com/
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>