platform: ariane-fpga: Set per-HART stack size to 8KB

Recent commit

  4a603eb6 ("platform: kendryte/k210: Set per-HART stack size to 8KB")

forgot to update ariane-fpga codes, and the following commit

  678c3c36

 ("include: sbi_scratch: Set per-HART scratch size to 4KB")

changed the per-HART scratch size to 4KB, which potentially breaks
ariane-fpga platform.

This patch set per-HART stack size of ariane-fpga to 8KB for consistency.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>