doc: sifive-fu540: Update README to explicitly load DTB for Linux

doc: sifive-fu540: Update README to explicitly load DTB for Linux

We should explicitly load DTB from TFTP server or MMC/SD card
for Linux booting. This will allow us:
1. To use different Linux DTB for SiFive Unleashed board with
   expansion board connected.
2. Avoid re-flashing OpenSBI firmware whenever board connections
   change.

This patch updates reference bootlog in SiFive FU540 README
as-per above.

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