mailbox: arm_mhu: add full support for the doorbells

We now have the basic infrastructure and binding to support doorbells
on ARM MHU controller.

This patch adds all the necessary mailbox operations to add support for
the doorbells. Maximum of 32 doorbells are supported on each physical
channel, however the total number of doorbells is restricted to 20
in order to save memory. It can increased if required in future.

Change-Id: I9788b68bdae97c60d5573c97d2057e531b0f3ba8
Cc: Alexey Klimov <alexey.klimov@arm.com>
Cc: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>