gpio: generic: provide helpers for reading and writing registers

gpio: generic: provide helpers for reading and writing registers

Provide helpers wrapping the read_reg() and write_reg() callbacks of the
generic GPIO API that are called directly by many users. This is done to
hide their implementation ahead of moving them into the separate generic
GPIO struct.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20250825-gpio-mmio-gpio-conv-v1-2-356b4b1d5110@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>