media: camss: Make ISPIF subdevice optional

This driver supports multiple architecture versions of the Qualcomm ISP.
The CAMSS architecure which this driver is name after, and with the
introduction of this series, the Titan architecture.

The ISPIF is an IP-block that is only present in the CAMSS generation of
the architecture. In order to support the Titan generation, make the
ISPIF an optional subdevice.

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>