include/configs: aspeed: Remove hardcoded variables

The hardcoded platform variables such as DRAM base address are not
common to Aspeed SoCs AST24xx/AST25xx/AST26xx. This patch replaces
those hardcoded with macros defined in a newly added header, where
the basic SoC HW information are assigned accordingly.

Signed-off-by: Chia-Wei, Wang <chiawei_wang@aspeedtech.com>