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 be91c36247089 ("dt-bindings:
add document for rockchip variant of analogix_dp").

Reported-by: Tomasz Figa <[e-mail]>
Signed-off-by: Yakir Yang <[e-mail]>
Reviewed-by: Douglas Anderson <[e-mail]>
Reviewed-by: Tomasz Figa <[e-mail]>

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

···
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 Required properties:
-- compatible: "rockchip,rk3288-edp",
+- compatible: "rockchip,rk3288-dp",
 	      "rockchip,rk3399-edp";
 
 - reg: physical base address of the controller and length