OMAP: voltage: move plat/voltage.h to mach-omap2/voltage.h

At this point in time, there's no reason for this header file to be in
plat-omap/include/plat/voltage.h.  It should not be included by device
drivers, and the code that uses it is currently all under mach-omap2/.

Signed-off-by: Paul Walmsley <paul@pwsan.com>