common: Move random-number functions into their own header

Create a new rand.h header file and move functions into it, to reduce
the size of common.h

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

lib/rand.c

@@ -8,6 +8,7 @@
  */
 
 #include <common.h>
+#include <rand.h>
 
 static unsigned int y = 1U;