media: camss: Refactor VFE HW version support

In order to support Qualcomm ISP hardware architectures that diverge
from older architectures, the VFE subdevice driver needs to be refactored
to better abstract the different ISP architectures.

Gen1 represents the CAMSS ISP architecture. The ISP architecture developed
after CAMSS, Titan, will be referred to as Gen2.

Signed-off-by: Robert Foss <robert.foss@linaro.org>
Reviewed-by: Andrey Konovalov <andrey.konovalov@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>