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 <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>