media: i2c: adv748x: add adv748x driver

Provide support for the ADV7481 and ADV7482.

The driver is modelled with 4 subdevices to allow simultaneous streaming
from the AFE (Analog front end) and HDMI inputs though two CSI TX
entities.

The HDMI entity is linked to the TXA CSI bus, whilst the AFE is linked
to the TXB CSI bus.

The driver is based on a prototype by Koji Matsuoka in the Renesas BSP,
and an earlier rework by Niklas Söderlund.

Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>