board: arbel: Limit the dram effective size to bank0 maximal size

For 4GB dram size, the dram is divided into 2 banks and
the address space of these 2 banks are not concatenated.
Limit the gd->ram_top to not exceed bank0 top to prevent
accessing invalid memory region.

Signed-off-by: Jim Liu <JJLIU0@nuvoton.com>