move CLI prototypes to cli.h and add comments

Move the CLI prototypes from common.h to cli.h as part of an effort to
reduce the size of common.h.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/cmd_i2c.c b/common/cmd_i2c.c
index ebce7d4..8ccde68 100644
--- a/common/cmd_i2c.c
+++ b/common/cmd_i2c.c
@@ -66,6 +66,7 @@
  */
 
 #include <common.h>
+#include <cli.h>
 #include <command.h>
 #include <edid.h>
 #include <environment.h>