[C72 Nano] [iMX8Mn] Add RAM 2GB support
requested to merge seco/develop/imx_5.4.70_2.3.0-sandrelli into seco/develop/imx_5.4.70_2.3.0-develop
add support to 2GB RAM
modified: board/seco/imx8mn_c72/spl.c
add support to 2GB RAM
modified: board/seco/imx8mn_c72/spl.c