gpio: mmio: get chip label and GPIO base from device properties

gpio: mmio: get chip label and GPIO base from device properties

Ahead of removing struct bgpio_pdata support from the gpio-mmio generic
module, let's add support for getting the relevant values from generic
device properties. "label" is a semi-standardized property in some GPIO
drivers so let's go with it. There's no standard "base" property, so
let's use the name "gpio-mmio,base" to tie it to this driver
specifically. The number of GPIOs will be retrieved using
gpiochip_get_ngpios() so there's no need to look it up in the software
node.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20250701-gpio-mmio-pdata-v2-2-ebf34d273497@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>