configs: raise SPL_SYS_MALLOC_SIZE to 8 MiB on RISC-V

configs: raise SPL_SYS_MALLOC_SIZE to 8 MiB on RISC-V

On several RISC-V boards we have seen that 1 MiB is a insufficient value
for CONFIG_SPL_SYS_MALLOC_SIZE.

For instance qemu-riscv32_spl_defconfig fails booting because u-boot.itb
exceeds 1 MiB.

8 MiB is a reasonable value that allows adding FPGA blobs or splash images
to main U-boot.

Reported-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>