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>
diff --git a/arch/arm/mach-omap2/sr_device.c b/arch/arm/mach-omap2/sr_device.c
index a636604..10d3c5e 100644
--- a/arch/arm/mach-omap2/sr_device.c
+++ b/arch/arm/mach-omap2/sr_device.c
@@ -23,9 +23,9 @@
 #include <linux/io.h>
 
 #include <plat/omap_device.h>
-#include <plat/voltage.h>
 
 #include "smartreflex.h"
+#include "voltage.h"
 #include "control.h"
 #include "pm.h"