drm: crc: Introduce get_crc_sources callback

This patch introduce a callback function "get_crc_sources" which
will be called during read of control node. It is an optional
callback function and if driver implements this callback, driver
should return a constant pointer to an array of crc sources list
and update count according to the number of source in the list.

Changes Since V1: (Daniel)
 - return const pointer to an array of crc sources list
 - do validation of sources in CRC-core
Changes Since V2:
 - update commit message
 - update callback documentation
 - print one source name per line

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Cc: dri-devel@lists.freedesktop.org
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180713135942.25061-3-mahesh1.kumar@intel.com