dts: nordic: nrf9160: Add IPC node

Add devicetree node for IPC driver so that the IPC interrupt properties
can be accessed with device tree interrupt property macros and
configured using devicetree overlays.

Signed-off-by: Eivind Jølsgard <eivind.jolsgard@nordicsemi.no>
Co-authored-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>