soc: nordic_nrf: nrf91: Add support for nRF9161 SiP / nRF9120 SoC

The nRF9161 is technically a SiP (System-in-Package) that consists of
the nRF9120 SoC and additional components like PMIC, FEM, and XTAL,
so for nrfx/MDK the nRF9120 SoC is to be selected as the build target,
but since the nRF9161 is what a user can actually see on a board, using
only nRF9120 in the Zephyr build infrastructure might be confusing.
That's why in the top level of SoC definitions (for user-configurable
options in Kconfig, for example) the nRF9161 term is used and nRF9120
underneath.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>