mailbox/omap: consolidate OMAP mailbox driver

There is no need for a separate common OMAP mailbox module
now that the OMAP1 mailbox driver has been removed. So,
consolidate the two individual OMAP mailbox modules into a
single driver. This streamlines the driver for converting
to mailbox framework.

The following are the main changes:
- collapse mailbox-omap2.c into omap-mailbox.c
- remove omap_mbox_ops and replace the ops calls with
  the equivalent functionality.
- simplify the sub-mailbox startup/shutdown functionality,
  the one-time operations are moved into probe, and the
  pm_runtime_get_sync and pm_runtime_put_sync can be invoked
  without using a configuration counter.
- move all definitions from private omap_mbox.h into the
  source code, and eliminate this internal header.
- rename some variables that used the omap2_mbox prefix with
  a generic omap_mbox prefix.

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