verdin-imx8mm/verdin-imx8mp: move imx verdins to OF_UPSTREAM

Move verdin-imx8mm and verdin-imx8mp to OF_UPSTREAM:
- handle the fact that dtbs now have a 'freescale/' prefix
- imply OF_UPSTREAM
- remove redundant files from arch/arm/dts leaving only the
  *-u-boot.dtsi files
- update MAINTAINERS files

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
diff --git a/arch/arm/mach-imx/imx8m/Kconfig b/arch/arm/mach-imx/imx8m/Kconfig
index 3823d21..23d9217 100644
--- a/arch/arm/mach-imx/imx8m/Kconfig
+++ b/arch/arm/mach-imx/imx8m/Kconfig
@@ -262,12 +262,14 @@
 	select IMX8MM
 	select SUPPORT_SPL
 	select IMX8M_LPDDR4
+	imply OF_UPSTREAM
 
 config TARGET_VERDIN_IMX8MP
 	bool "Support Toradex Verdin iMX8M Plus module"
 	select IMX8MP
 	select SUPPORT_SPL
 	select IMX8M_LPDDR4
+	imply OF_UPSTREAM
 
 config TARGET_IMX8MM_BEACON
 	bool "imx8mm Beacon Embedded devkit"