arch: arm: nrf: Rename common header to apply to all nRFx ICs

The existing nrf5_common.h now applies to other Nordic ICs that are not
part of the "5" family. Instead rename this to nrf_common.h to cover the
upcoming ICs that belong to other families.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>