mailbox: arm_mhuv2: Add driver

This adds driver for the ARM MHUv2 (Message Handling Unit) mailbox
controller.

This is based on the accepted DT bindings of the controller and supports
combination of both transport protocols, i.e. doorbell and data-transfer.

Transmitting and receiving data through the mailbox framework is done
through struct arm_mhuv2_mbox_msg.

Based on the initial work done by Morten Borup Petersen from ARM.

Co-developed-by: Tushar Khandelwal <tushar.khandelwal@arm.com>
Signed-off-by: Tushar Khandelwal <tushar.khandelwal@arm.com>
Tested-by: Usama Arif <usama.arif@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>