common: Move sorting functions to their own header file

These don't need to be in common.h so move them out into a new header.
Also add some missing comments.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>

lib/qsort.c

@@ -18,6 +18,7 @@
 #include <linux/types.h>
 #include <common.h>
 #include <exports.h>
+#include <sort.h>
 
 void qsort(void  *base,
 	   size_t nel,