media: uapi: Add an entity type for Image Signal Processors

Add and document a media entity type for Image Signal Processor devices.
Surprisingly we didn't have one, so add one now. More or less all ISP
drivers should use this type instead of what they currently are using (or
not using anything).

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>