nds32: Enable two banks of SDRAM on Andes board

The original adp-ag101/adp-ag101p initialize only one bank(64MB)
by default at boot time, but it is not enough for some application,
so increasing to two banks(128M).

Signed-off-by: Kuan-Yu Kuo <ken.kuoky@gmail.com>
Cc: Macpaul Lin <macpaul@gmail.com>