media: ccs-pll: Document the CCS PLL flags

Document the CCS PLL flags with short comments. The CCS spec has more
information on them while the added documentation helps finding the
relevant information in the CCS spec.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>