gpio: regmap: Group optional assignments together for better understanding

Group ngpio_per_reg, reg_stride, and reg_mask_xlate assignments together
with the respective conditional for better understanding what's going on
in the code.

While at it, mark ngpio_per_reg as (Optional) in the kernel-doc
in accordance with what code actually does.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Michael Walle <mwalle@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Reviewed-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Link: https://lore.kernel.org/r/20250213195621.3133406-4-andriy.shevchenko@linux.intel.com


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>