Unify set of the pins on `nrf52840dk_nrf52840`, which is used by
`gpio_loopback` fixture.
Signed-off-by: Katarzyna Giądła <katarzyna.giadla@nordicsemi.no>
Added test which validates that uart device correctly handles
going to and from low power state.
Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>