gpio: xilinx: Use better bitmap APIs where appropriate

gpio: xilinx: Use better bitmap APIs where appropriate

There are bitmap_gather() and bitmap_scatter() that are factually
simplified version of the APIs used in the driver. Use them where
appropriate.

While at it, replace bitmap_bitremap() with find_nth_bit()
for the sake of simplification.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20250205093200.373709-2-andriy.shevchenko@linux.intel.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>