ARM: dts: koelsch: Move cec_clock to root node

cec-clock is a fixed clock generator that is not controlled by i2c5 and
thus should not be a child of the i2c5 bus node. Rather, it should be
a child of the root node of the DT.

Fixes: 02a5ab18

 ("ARM: dts: koelsch: Add CEC clock for HDMI transmitter")
Reported-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>