add platform C72 Mini support

	modified:   arch/arm/dts/Makefile
	new file:   arch/arm/dts/seco-imx8mm-c72-u-boot.dtsi
	new file:   arch/arm/dts/seco-imx8mm-c72.dts
	modified:   arch/arm/mach-imx/imx8m/Kconfig
	modified:   board/seco/common/Makefile
	new file:   board/seco/common/seco_imx8mm_c72_setup.c
	new file:   board/seco/imx8mm_c72/Kconfig
	new file:   board/seco/imx8mm_c72/Makefile
	new file:   board/seco/imx8mm_c72/ddr4_timing.c
	new file:   board/seco/imx8mm_c72/env_conf.h
	new file:   board/seco/imx8mm_c72/imx8mm_c72.c
	new file:   board/seco/imx8mm_c72/spl.c
	new file:   configs/seco_imx8mm_c72_defconfig
	new file:   include/configs/seco_imx8mm_c72.h
	modified:   include/configs/seco_mx8_dtbo.h
	new file:   include/seco/seco_imx8mm_c72_setup.h