dt-bindings: mailbox: arm,mhuv3: Add bindings

Add bindings for the ARM MHUv3 Mailbox controller.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>