media: sun6i-csi: Detect the availability of the ISP

Add a helper to detect whether the ISP is available and connected
and store the indication in the driver-specific device structure.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>