docs: Remove out-of-date documentation

Upstream U-Boot now have SMP support and doesn't require any additional
patches for HiFive Unleashed.

Update the documentation.

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