ARM: dts: msm: Add mailbox controller support for NPU

The mailbox controller implemented in NPU works as a bridge
between ipcc_mproc mailbox controller and its clients. It is
required because npu driver needs to control when IPCC irq can
be sent to NPUQ6.

Change-Id: I2987c1820169577c6985fa922e2401f865f13b7b