mailbox/omap: remove omap_mbox_type_t from mailbox ops

The type definition omap_mbox_type_t used for distinguishing
OMAP1 from OMAP2+ mailboxes is no longer needed after the
removal of OMAP1 mailbox driver, and has therefore been
cleaned up. This cleanup also eliminates the need for the
polling logic used for checking the transmit readiness.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>