Add DT bindings documentation for the ARM display processor D71 and later
IPs.

Changes in v4:
- Deleted unnecessary address-cells, size-cells [Liviu Dudau]

Changes in v3:
- Deleted unnecessary property: interrupt-names.
- Dropped 'ports' and moving 'port' up a level.
Signed-off-by: James Qian Wang (Arm Technology China) <james.qian.wang@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
[moved in the same directory as the other Arm display bindings]
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>