media: cec: extron-da-hd-4k-plus: add the Extron DA HD 4K Plus CEC driver

Add support for the Extron DA HD 4K Plus series of 4K HDMI
Distrubution Amplifiers (aka HDMI Splitters).

These devices support CEC and this driver adds support for the
CEC protocol for both the input and all outputs (2, 4 or 6 outputs,
depending on the model).

It also exports the EDID from the outputs and allows reading and
setting the EDID of the input.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>