OMAP4: clocks: add dummy clock for mailbox

In omap4, there is no explicit configuration register to enable mailbox clocks.
Defining dummy clock for mailbox clock module to keep the mailbox driver
backward compatible with previous omaps.

Signed-off-by: Hari Kanigeri <h-kanigeri2@ti.com>
Acked-by: Benoît Cousson <b-cousson@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>