soc: arm: nordic_nrf: replace NRF_DT_CHECK_PIN_ASSIGNMENTS

Since PINCTRL and pinctrl-0 is now required, there's no point in doing
extra validation at driver level. Modify the macro to just check that
sleep state is present when needed, since it was the only remaining
assertion that was not covered. Renamed the macro to make it more clear
what it does: NRF_DT_CHECK_NODE_HAS_PINCTRL_SLEEP

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