media: cec-notifier: add cec_notifier_parse_hdmi_phandle helper

Add helper function to parse the DT for the hdmi-phandle property
and return the corresponding struct device pointer.

It takes care to avoid increasing the device refcount since all
we need is the device pointer. This pointer is used in the
notifier list as a key, but it is never accessed by the CEC driver.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reported-by: Wen Yang <wen.yang99@zte.com.cn>
Acked-by: Wen Yang <wen.yang99@zte.com.cn>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>