Merge C20 on develop
add c20 board derived from nxp imx8mq_val
modified: arch/arm/dts/Makefile
new file: arch/arm/dts/seco-imx8mq-c20.dts
modified: arch/arm/mach-imx/imx8m/Kconfig
modified: board/seco/common/Makefile
modified: board/seco/common/mmc.c
new file: board/seco/imx8mq_c20/Kconfig
new file: board/seco/imx8mq_c20/Makefile
new file: board/seco/imx8mq_c20/ddr/ddr.h
new file: board/seco/imx8mq_c20/ddr/ddr3l/ddr_init.c
new file: board/seco/imx8mq_c20/ddr/ddr3l/ddrphy_train.c
new file: board/seco/imx8mq_c20/ddr/helper.c
new file: board/seco/imx8mq_c20/ddr/wait_ddrphy_training_complete.c
new file: board/seco/imx8mq_c20/ddr4_timing.c
new file: board/seco/imx8mq_c20/imx8mq_c20.c
new file: board/seco/imx8mq_c20/spl.c
new file: configs/seco_imx8mq_c20_defconfig
new file: include/configs/seco_imx8mq_c20.h