riscv: ae350: Adjust the memory layout of ae350

Adjust the initial stack pointer address to 0x10000000(256M)

Signed-off-by: Leo Yu-Chi Liang <ycliang@andestech.com>
Reviewed-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Rick Chen <rick@andestech.com>