arm64: dts: rockchip: Simplify edp endpoints on several rk3399 boards

arm64: dts: rockchip: Simplify edp endpoints on several rk3399 boards

The only thing actually added here is a single endpoint on edp_out,
which is already defined in rk3399-base.dtsi, so it's simpler to just
reference that phandle, which allows the removal of several properties.

This fixes the following DTB validation warnings:

  graph node has single child node 'endpoint@0',
  #address-cells/#size-cells are not necessary

Signed-off-by: Diederik de Haas <didi.debian@cknow.org>
Link: https://lore.kernel.org/r/20250709132323.128757-6-didi.debian@cknow.org
Signed-off-by: Heiko Stuebner <heiko@sntech.de>