Move strtomhz() to vsprintf.h

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>