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

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

Based on a similar patch for Koelsch by Hans Verkuil.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>