mailbox/omap: kill omap_mbox_{save/restore}_ctx() functions

The omap_mbox_save_ctx() and omap_mbox_restore_ctx() API were
previously provided to OMAP mailbox clients to save and restore
the mailbox context during system suspend/resume. The save and
restore functionality is now implemented through System PM driver
callbacks, and there is no need for these functions, so kill these
API.

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