dts: xtensa: nxp: add nodes for IPC

Add mailbox and interrupt-controller nodes used for
inter-process communication.

Add also the dt binding for the interrupt-controller.
For now, this is used just to fix some compile errors,
since the mailbox requires an interrupt-controller.

For DSP, we have a direct interrupt line to the core.

Signed-off-by: Iuliana Prodan <iuliana.prodan@nxp.com>