mailbox/omap: add support for suspend/resume

Support has been added to the OMAP mailbox driver to allow it
to work across a system suspend/resume. The OMAP mailbox driver
requires only the interrupt configuration registers to be saved
and restored, and this is done in the suspend/resume callbacks.
The registers need to be saved only if there are active clients
at the time of suspend. The enabling and disabling of the mailbox
clocks is done automatically by the omap_device layer.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>