doc: sifive-fu540: Update README for steps to create FW_PAYLOAD

Due changes in DT bindings, we now embed DTB from Linux-5.3 (or
higher) in OpenSBI FW_PAYLOAD along with payload u-boot.bin. This
patch updates SiFive FU540 README to reflect the changes in build
and boot steps.

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