mailbox: add support for ARM Message Handling Unit(MHU) controller

mailbox: add support for ARM Message Handling Unit(MHU) controller

This patch adds support for ARM Message Handling Unit(MHU) controller
that provides control logic and interrupt generation to support
inter-processor communication between the Application Processor and
the System Control Processor(SCP).

This support is built on the existing common mailbox framework for
client/protocol drivers and controller drivers of Inter Processor
Communication(IPC). SCP controls most of the power management on the
Application Processors.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>