configs: stratix10: Change CONFIG_NR_DRAM_BANKS to 2

Stratix10 maps dram in 2 address spans, from 0-2GB and from
2GB up to 128GB.

Signed-off-by: Dalon Westergreen <dalon.westergreen@intel.com>
Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>