media: rcar-csi2: Prepare for Gen4 support

Prepare the driver for supporting R-Car Gen4. The starting of the
receiver and how to enter standby differs between Gen3 and Gen4,
create function pointers in the device info structure to control the
different behavior.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>