platform: Remove the HiFive Unleashed expansion board DT.

The Linux kernel will maintain the device tree (DT) for HiFive Unleashed
from release v5.2-rc6. This DT is incompatible with previous DT
present for Microsemi expansion board in OpenSBI.

Since, OpenSBI will directly load the pre-built DTB from kernel now onwards,
no need to keep the out-of-date DT in openSBI. Remove this DT and
it's related documentation.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Acked-by: Anup Patel <anup.patel@wdc.com>