dt-bindings: mailbox : arm,mhuv2: Add bindings

This patch adds device tree binding for ARM Message Handling Unit (MHU)
controller version 2.

Based on earlier work by Morten Borup Petersen.

Reviewed-by: Rob Herring <robh@kernel.org>
Co-developed-by: Tushar Khandelwal <tushar.khandelwal@arm.com>
Signed-off-by: Tushar Khandelwal <tushar.khandelwal@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>