gpio: xgene-sb: use generic GPIO chip register read and write APIs

gpio: xgene-sb: use generic GPIO chip register read and write APIs

The conversion to using the modernized generic GPIO chip API was
incomplete without also converting the direct calls to write/read_reg()
callbacks. Use the provided wrappers from linux/gpio/generic.h.

Fixes: 38d98a822c14 ("gpio: xgene-sb: use new generic GPIO chip API")
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20250910-gpio-mmio-gpio-conv-part4-v2-6-f3d1a4c57124@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>