drm/komeda: Add support for 'memory-region' DT node property

The 'memory-region' property of the komeda display driver DT binding
allows the use of a 'reserved-memory' node for buffer allocations. Add
the requisite of_reserved_mem_device_{init,release} calls to actually
make use of the memory if present.

Changes since v1:
 - Move handling inside komeda_parse_dt

Signed-off-by: Mihail Atanassov <mihail.atanassov@arm.com>
Reviewed-by: James Qian Wang (Arm Technology China) <james.qian.wang@arm.com>
Signed-off-by: james qian wang (Arm Technology China) <james.qian.wang@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190805095408.21285-1-mihail.atanassov@arm.com