power: supply: gpio_charger: switch to using GPIO descriptors
The GPIO charger is using a mix of the legacy GPIO interface
and <linux/of_gpio.h> which is not the modern way to use GPIOs.
Refactor like this:
- Use a GPIO descriptor for the GPIO line used to monitor the
charger.
- Fetch the descriptor with devm_gpiod_get() as the first
method.
- If this fails and we are *not* using device tree, then
start looking to see if we can use platform data instead.
- After looking up and requesting a GPIO number with the
legacy API, convert it to a descriptor.
This way we can later isolate and drop the legacy code as
more platforms move over to using descriptors.