media: i2c: ds90ub960: Configure serializer using back-channel

For DS90UB9702-Q1, it is recommended to configure some serializer
settings over the back-channel before the forward-channel is active.

This can only be done if the serializer's I2C address on the FPD-Link
bus is populated in the device tree node.

Signed-off-by: Jai Luthra <jai.luthra@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>