ARM: dts: imx: Switch to using upstream DT on DH i.MX6 DHCOM

Enable OF_UPSTREAM to use upstream DT and add nxp/imx/ prefix to the
DEFAULT_DEVICE_TREE. And thereby directly build DTB from dts/upstream/src/
including *-u-boot.dtsi files from arch/$(ARCH)/dts/ directory.

Signed-off-by: Marek Vasut <marex@denx.de>
diff --git a/arch/arm/dts/Makefile b/arch/arm/dts/Makefile
index aeccfa9..6ecc7bb 100644
--- a/arch/arm/dts/Makefile
+++ b/arch/arm/dts/Makefile
@@ -778,8 +778,6 @@
 	imx6dl-cubox-i.dtb \
 	imx6dl-cubox-i-emmc-som-v15.dtb \
 	imx6dl-cubox-i-som-v15.dtb \
-	imx6dl-dhcom-pdk2.dtb \
-	imx6dl-dhcom-picoitx.dts \
 	imx6dl-gw51xx.dtb \
 	imx6dl-gw52xx.dtb \
 	imx6dl-gw53xx.dtb \
@@ -811,8 +809,7 @@
 	imx6dl-sabreauto.dtb \
 	imx6dl-sabresd.dtb \
 	imx6dl-sielaff.dtb \
-	imx6dl-wandboard-revd1.dtb \
-	imx6s-dhcom-drc02.dtb
+	imx6dl-wandboard-revd1.dtb
 
 endif
 
@@ -824,7 +821,6 @@
 	imx6q-cubox-i.dtb \
 	imx6q-cubox-i-emmc-som-v15.dtb \
 	imx6q-cubox-i-som-v15.dtb \
-	imx6q-dhcom-pdk2.dtb \
 	imx6q-display5.dtb \
 	imx6q-gw51xx.dtb \
 	imx6q-gw52xx.dtb \