docs: Update the fu540 platform guide as per U-Boot documents.

U-Boot readme for fu540 platform suggest that fdt_addr_r should be used
as DT address after DT is copied via tftpboot.

Update the OpenSBI docs to reflect that. Remove other stale informations
as well.

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