On page SEEED SOM STM32MP157C - Seeed Wiki
The listed RAM on the board is incorrect. The page says it has 512Mb but the data sheet says that part is a 4Gb chip. https://www.micron.com/products/dram/ddr3-sdram/part-catalog/mt41k256m16tw-107-it .
512MB = 4Gb 
Cheers,
Ben