drm/i2c: tda998x: add CEC support

The TDA998x is a HDMI transmitter with a TDA9950 CEC engine integrated
onto the same die.  Add support for the TDA9950 CEC engine to the
TDA998x driver.

Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>