docs: platform: Describe sifive_fu540 as supported generic platform

The upstream U-Boot/QEMU have been using generic platform for SiFive
HiFive Unleashed board for some time. Let's document sifive_fu540 as
one of the supported targets for "generic" platform.

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