ARMV7: OMAP: Add detection and support for Beagle C4 revision

This patch enhances the revision detection function and adds
support for the C4 revision.  The board revision is printed
and approriate revision specific setup is done automatically.

Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>