dt-bindings: analogix_dp: rockchip: correct the wrong compatible name

The document about rockchip platform make a mistaken in available
compatible name of "rk3288-edp", we should correct it to "rk3288-dp"
which correspond to the compatible name in driver.

This mistaken was introduced in commit be91c362 ("dt-bindings:
add document for rockchip variant of analogix_dp").
Reported-by: Tomasz Figa <tfiga@chromium.com>
Signed-off-by: Yakir Yang <ykk@rock-chips.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Tomasz Figa <tomasz.figa@chromium.com>

Documentation/devicetree/bindings/display/rockchip/analogix_dp-rockchip.txt

@@ -2,7 +2,7 @@ Rockchip RK3288 specific extensions to the Analogix Display Port
 ================================
 
 Required properties:
-- compatible: "rockchip,rk3288-edp",
+- compatible: "rockchip,rk3288-dp",
 	      "rockchip,rk3399-edp";
 
 - reg: physical base address of the controller and length