drm/afbc: Add AFBC modifier usage documentation

AFBC is a flexible, proprietary, lossless compression protocol and
format, with a number of defined DRM format modifiers. To facilitate
consistency and compatibility between different AFBC producers and
consumers, document the expectations for usage of the AFBC DRM format
modifiers in a new .rst chapter.

Signed-off-by: Brian Starkey <brian.starkey@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
[Updated MAINTAINERS entry to show that "Mali DP Maintainers" is
 actually a mailing list and added an SPDX-License-Identifier to
 the documentation]
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>

include/uapi/drm/drm_fourcc.h

@@ -572,6 +572,9 @@ extern "C" {
  * AFBC has several features which may be supported and/or used, which are
  * represented using bits in the modifier. Not all combinations are valid,
  * and different devices or use-cases may support different combinations.
+ *
+ * Further information on the use of AFBC modifiers can be found in
+ * Documentation/gpu/afbc.rst
  */
 #define DRM_FORMAT_MOD_ARM_AFBC(__afbc_mode)	fourcc_mod_code(ARM, __afbc_mode)