mailbox: arm_mhu: add support to read and record mbox-name

It's sometimes useful to identify the mailbox controller with the name
as specified in the devicetree via mbox-name property especially in a
system with multiple controllers.

This patch adds support to read and record the mailbox controller name.

Change-Id: I50cf79bf3b68b2f74d38c29337db30e125fa2e87
Cc: Alexey Klimov <alexey.klimov@arm.com>
Cc: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>