OMAP2+: voltage: split out voltage processor (VP) code into new layer

This patch is primarily a move of VP specific code from voltage.c into
its own code in vp.c and adds prototypes to vp.h

No functional changes, except debugfs...

VP debugfs moved to 'vp' subdir of <debugfs>/voltage/ and 'vp_'
prefixes removed from all debugfs filenames.

Signed-off-by: Kevin Hilman <khilman@ti.com>