Move strtomhz() to vsprintf.h

At present this function sits in its own file but it does not really
justify it. There are similar string functions in vsprintf.h, so move it
there. Also add the missing function comment.

Use the vsprintf.h include file explicitly where needed.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/board/freescale/p1_twr/ddr.c b/board/freescale/p1_twr/ddr.c
index 7e8bd6b..85f1f63 100644
--- a/board/freescale/p1_twr/ddr.c
+++ b/board/freescale/p1_twr/ddr.c
@@ -4,6 +4,7 @@
  */
 
 #include <common.h>
+#include <vsprintf.h>
 #include <asm/mmu.h>
 #include <asm/immap_85xx.h>
 #include <asm/processor.h>