ARM: OMAP2+: mbox: remove dependencies with soc.h

The OMAP mailbox platform driver code has been cleaned up to
remove the dependencies with soc.h in preparation for moving
the mailbox code to drivers folder.

The code relied on cpu_is_xxx/soc_is_xxx macros previously to
pick the the right set of mailbox devices and register with the
mailbox driver. This data is now represented in a concise format
and moved to the respective omap_hwmod data files and published
to the driver through the platform data.

Cc: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Suman Anna <s-anna@ti.com>