media: sun6i-csi: Define and use variant to get module clock rate

Introduce a proper variant structure with the module clock rate
instead of hardcoding it with a manual check on the compatible.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>