board: gateworks: venice: determine dram size at runtime

The SPL does not update the memory node with the dram size from EEPROM
but instead we can use get_ram_size which does a simple memory test
to determine the available RAM. Update PHYS_SDRAM_SIZE to 4GiB as that
is the max used on the Venice boards.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>