ARM: dts: Disable HDMI CEC internal pull-ups

Devices using an external encoder, ESD protection and level shifter
such as tpd12s015 or ip4791cz12 have the CEC pull in the encoder
chip. And on var-som-om44, there is external pull up resistor R30.

So the internal CEC pull-up resistor needs to be disabled as otherwise
the external and internal pull are parallel making the pull value
much smaller than intended. This leads into the CEC not working as
reported by Hans Verkuil <hverkuil@xs4all.nl>.

Reported-by: Hans Verkuil <hverkuil@xs4all.nl>
Cc: Dmitry Lifshitz <lifshitz@compulab.co.il>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>