gpio: mmio: Add flag for calling pinctrl back-end

It turns out that with this flag we can switch over an entire
driver to use gpio-mmio instead of a bunch of custom code,
also providing get/set_multiple() to it in the process, so it
seems like a reasonable feature to add.

The generic pin control backend requires us to call the
gpiochip_generic_request(), gpiochip_generic_free(),
pinctrl_gpio_direction_output() and pinctrl_gpio_direction_input()
callbacks, so if the new flag for a pin control back-end
is set, we make sure these functions get called as
expected.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20250219-vf610-mmio-v3-1-588b64f0b689@linaro.org


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