arch: arm64: dts: librem5-devkit: disable hdmi and dcss
requested to merge martin.kepplinger/linux-next:5.6-rc5/librem5__integration into imx8-linux-next-librem5
a MR for completeness, see https://source.puri.sm/Librem5/linux-next/commit/122fdd7a7eb2b031bb8b2a5b65e312d7f9625ad8