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>

arch/arm/mach-omap2/pm.c

@@ -18,8 +18,8 @@
 #include <plat/omap-pm.h>
 #include <plat/omap_device.h>
 #include <plat/common.h>
-#include <plat/voltage.h>
 
+#include "voltage.h"
 #include "powerdomain.h"
 #include "clockdomain.h"
 #include "pm.h"