mailbox: omap: Move omap_mbox_irq_t into driver

This is only used internal to the driver, move it out of the
public header and into the driver file. While we are here,
this is not used as a bitwise, so drop that and make it a
simple enum type.

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