sun4i_hdmi: add CEC support

Add HDMI CEC support to the Allwinner A10 SoC.

This SoC uses a poor-man's CEC implementation by polling the CEC pin. It is
using the CEC_PIN core implementation for such devices to do the heavy
lifting. It just provides the callbacks to read/drive the CEC pin.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Tested-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>