riscv: ae350: Increase malloc size for binman spl flow

riscv: ae350: Increase malloc size for binman spl flow

It will need larger heap size for u-boot-spl to load u-boot.itb which
be generated from binman than USE_SPL_FIT_GENERATOR.

Signed-off-by: Rick Chen <rick@andestech.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>