uboot-imx

riscv: Enlarge the default SYS_MALLOC_F_LEN

Increase the heap size for the pre-relocation stage, so that CPU
driver can be loaded.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Anup Patel <anup@brainfault.org>