docs: platform: Update SiFive FU540 doc as-per U-Boot v2020.01

With U-Boot v2020.01, the SiFive FU540 DTB required by U-Boot is
embedded in U-Boot binary itself so we don't need to do anything
special for U-Boot v2020.01 as payload to OpenSBI firmware.

This patch updates SiFive FU540 documenation assuming we use
latest U-Boot v2020.01 release.

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