input: gpio_keys: implement polling mode support

Some MCU have limitations with GPIO interrupts. Add a polling mode to
the gpio-keys driver to support those cases.

This required a bit of a refactoring of the driver data structure to add
a instance wide data, and move the pin specific pointer in the config
structure.

For polling, reuse the button 0 delayed work so we minimize the resource
waste, the two work handler functions are only referenced when used so
at least those are discarded automatically if no instance needs them.

Fix a bug in the PM structure instantiation as well.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>