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>

include/vsprintf.h

@@ -212,4 +212,14 @@ void print_grouped_ull(unsigned long long int_val, int digits);
 
 bool str2off(const char *p, loff_t *num);
 bool str2long(const char *p, ulong *num);
+
+/**
+ * strmhz() - Convert a value to a Hz string
+ *
+ * This creates a string indicating the number of MHz of a value. For example,
+ * 2700000 produces "2.7".
+ * @buf: Buffer to hold output string, which must be large enough
+ * @hz: Value to convert
+ */
+char *strmhz(char *buf, unsigned long hz);
 #endif