input: unify gpio-keys and zephyr,gpio-keys

Change the gpio-keys and zephyr,gpio-keys so that they can both be used
with the input subsystem driver. Make the zephyr,code property optional
so that existing out of tree board can still use this node with their
custom code, but change everything else so that an existin gpio-keys
node can be used with the input driver as long as the codes are defined.

From the application perspective, this means that the application can
still use the GPIOs directly, the input specific driver only gets
enabled if CONFIG_INPUT is enabled and the driver can always be turned
off manually.

This makes gpio-keys behave the same as gpio-leds with CONFIG_LED.

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