FROMGIT: dt-bindings: mediatek: add description for postmask

add description for postmask
postmask is used control round corner for display frame

Signed-off-by: Yongqiang Niu <yongqiang.niu@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

(cherry picked from commit 71aacdfce044d6136e427c7aaf1715d13ed9f57c
https://git.kernel.org/pub/scm/linux/kernel/git/chunkuang.hu/linux.git mediatek-drm-next)

BUG=b:153619806
TEST=emerge-kukui chromeos-kernel-5_10

Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Change-Id: I3b9e933c0003d78e51dba8703f41fa57e0787174
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/2639732
Commit-Queue: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/2644676
diff --git a/Documentation/devicetree/bindings/display/mediatek/mediatek,disp.txt b/Documentation/devicetree/bindings/display/mediatek/mediatek,disp.txt
index d6f03ad..b47e1a0 100644
--- a/Documentation/devicetree/bindings/display/mediatek/mediatek,disp.txt
+++ b/Documentation/devicetree/bindings/display/mediatek/mediatek,disp.txt
@@ -37,6 +37,7 @@
 	"mediatek,<chip>-disp-aal"   		- adaptive ambient light controller
 	"mediatek,<chip>-disp-gamma" 		- gamma correction
 	"mediatek,<chip>-disp-merge" 		- merge streams from two RDMA sources
+	"mediatek,<chip>-disp-postmask" 	- control round corner for display frame
 	"mediatek,<chip>-disp-split" 		- split stream to two encoders
 	"mediatek,<chip>-disp-ufoe"  		- data compression engine
 	"mediatek,<chip>-dsi"        		- DSI controller, see mediatek,dsi.txt