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/omap_twl.c b/arch/arm/mach-omap2/omap_twl.c
index 00e1d2b..ad8c18a 100644
--- a/arch/arm/mach-omap2/omap_twl.c
+++ b/arch/arm/mach-omap2/omap_twl.c
@@ -18,7 +18,7 @@
 #include <linux/kernel.h>
 #include <linux/i2c/twl.h>
 
-#include <plat/voltage.h>
+#include "voltage.h"
 
 #include "pm.h"