ARM: imx6q: add missing sentinel to divider table

The clk_enet_ref_table[] is missing a final empty entry as end of list
marker. Also make the existing markers more obvious.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>