docs/platform: sifive_fu540: Update U-Boot defconfig name

With latest U-Boot upstream (v2021.07 in development), the defconfig
name has been changed to sifive_unleashed_defconfig. Update the doc.

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