ast2600: spl: Locate load buffer in DRAM space

Return CONFIG_SYS_LOAD_ADDR pointing to DRAM space for
spl_get_load_buffer() to allow generic SPL image loading
code (e.g. FIT and Ymodem) to store data in DRAM.

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