arm64: dts: librem5: Drop separte DP device tree

Since we only power up the DP parts on plug we can use it by
default.

Signed-off-by: Guido Günther <guido.gunther@puri.sm>
2 jobs for mrs/5.9/dp-via-extcon in 67 minutes and 10 seconds
Status Name Job ID Coverage
  Build
passed build:aarch64 #307446
librem5

00:20:23

passed build:deb #307447
librem5:arm64

01:07:10