ram: aspeed: Add AST2600 DRAM control support

AST2600 supports DDR4 SDRAM with maximum speed DDR4-1600.
The DDR4 DRAM types including 128MbX16 (2Gb), 256MbX16 (4Gb),
512MbX16 (8Gb), 1GbX16 (16Gb), and 1GbX8 TwinDie (16Gb) are supported.

Signed-off-by: Dylan Hung <dylan_hung@aspeedtech.com>
Signed-off-by: Chia-Wei, Wang <chiawei_wang@aspeedtech.com>
Reviewed-by: Ryan Chen <ryan_chen@aspeedtech.com>