aspeed: ast2600: Enlarge SRAM size

The AST2600 SRAM has been extended to 88KB since A1
chip revision. This patch updates the SRAM size to
offer more space for early stack/heap use.

Signed-off-by: Chia-Wei Wang <chiawei_wang@aspeedtech.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>