arm: imx8mm-phycore: move to OF_UPSTREAM

The PHYCORE_IMX8MM is used by the phyBOARD-Polis and the
phyGATE-Tauri-L. Migrate both boards to OF_UPSTREAM. Linux kernel device
trees for both boards can be used as is, corresponding U-Boot device
tree files are removed. U-Boot tweaks are kept unchanged.

Signed-off-by: Yannic Moog <y.moog@phytec.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Teresa Remmet <t.remmet@phytec.de>
diff --git a/arch/arm/mach-imx/imx8m/Kconfig b/arch/arm/mach-imx/imx8m/Kconfig
index 23d9217..f81be42 100644
--- a/arch/arm/mach-imx/imx8m/Kconfig
+++ b/arch/arm/mach-imx/imx8m/Kconfig
@@ -296,6 +296,7 @@
 	select IMX8MM
 	select SUPPORT_SPL
 	select IMX8M_LPDDR4
+	imply OF_UPSTREAM
 
 config TARGET_PHYCORE_IMX8MP
 	bool "PHYTEC PHYCORE i.MX8MP"