soc: arm: nordic_nrf: move soc_nrf_common.h to common folder

Since the addition of pinctrl the common folder is added to the Zephyr
include path. This can be re-used to place the soc_nrf_common.h header
in the common folder and include it directly, without the need of
relative paths.

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