mailbox: arm_mhu: migrate to threaded irq handler

In preparation to introduce support for doorbells which require the
interrupt handlers to be threaded, this patch moves the existing
interrupt handler to threaded handler.

Also it moves out the registering and freeing of the handlers from
the mailbox startup and shutdown methods. This also is required to
support doorbells.

Change-Id: I8d7d826fa3b169cdccf7673ea94d7edb4af91c88
Cc: Alexey Klimov <alexey.klimov@arm.com>
Cc: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>