drm/i2c: tda9950: add CEC driver

Add a CEC driver for the TDA9950, which is a stand-alone I2C CEC device,
but is also integrated into HDMI transceivers such as the TDA9989 and
TDA19989.

The TDA9950 contains a command processor which handles retransmissions
and the low level bus protocol.  The driver just has to read and write
the messages, and handle error conditions.

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