mailbox: omap: Remove unneeded header omap-mailbox.h

The type of message sent using omap-mailbox is always u32. The definition
of mbox_msg_t is uintptr_t which is wrong as that type changes based on
the architecture (32bit vs 64bit). This type should have been defined as
u32. Instead of making that change here, simply remove the header usage
and fix the last couple users of the same in this driver.

Signed-off-by: Andrew Davis <afd@ti.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>