gpiolib: of: Integrate of_gpiochip_init_valid_mask() into gpiochip_init_valid_mask()

In preparation to complete fwnode switch, integrate
of_gpiochip_init_valid_mask() into gpiochip_init_valid_mask().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>