media: rcar-csi2: Use standby mode instead of resetting

Later versions of the datasheet updates the reset procedure to more
closely resemble the standby mode. Update the driver to enter and exit
the standby mode instead of resetting the hardware before and after
streaming is started and stopped. This replaces the software reset
(SRST.SRST) control.

While at it break out the full start and stop procedures from
rcsi2_s_stream() into the existing helper functions.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>