docs: Update the fu540 platform guide for new DT.

With 5.2-rc6 release, Linux kernel hosts the DT for Unleashed board
which is incompatible with the default DT from FSBL.

Update the document to use the DT from kernel.

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