dts: bindings: gpio-keys: align with Linux

gpio-keys binding has been used in Zephyr to describe random push
buttons in boards. On Linux, it is used by a keyboard input driver,
while in Zephyr is, in most cases, used directly in applications out of
the device driver model. This should likely be _fixed_ if one day we
have a proper input device class. Still, we can align a few things:

- Inherit from base.yaml (we need status, compatible, etc defined there)
- Remove the requirement of a label in child nodes
- Fix child nodes label description
- Move to dts/bindings/input

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>