ARM: dts: koelsch: Add CEC clock for HDMI transmitter

The adv7511 on the Koelsch board has a 12 MHz fixed clock
for the CEC block. Specify this in the dts to enable CEC support.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>