drivers: power_domain: introduce nrf_gpio_pad_group

Introduce the NRF GPIO Pad Group device driver and binding. The
pad group device represents the GPIO pads (pins), contrary to a
GPIO controller, which is one of the many devices which can be
muxed to pads in the pad group.

The pad group belong to a power domain, which is not neccesarily the
same power domain as devices being muxed to the pads, like GPIO or
UART. If no ACTIVE device is using any of the pads in the pad
group, the pad groups power domain may be SUSPENDED. Before the pad
groups power domain is SUSPENDED, pad config retention must be
enabled to prevent the pads from loosing their state. That's what
this device driver manages. Once retained, the pad configs and
outputs are locked, even when their power domain is SUSPENDED.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>