soc: common: nordic_nrf: move pinctrl_soc.h to a common dir

Because both, RISC-V and ARM cores share the same pinctrl driver. The
top level common folder will disappear with the introduction of HWMv2,
where multi-arch SoCs will be well supported.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>