docs/platform: sifive_fu540: Update U-Boot instructions

U-Boot v2020.07 release adds SPL support to SiFive HiFive Unleashed.
Update the doc to mention that detailed build instructions are in the
U-Boot doc.

This also adds detailed command line description to show how to boot
U-Boot v2020.07 S-mode payload in the QEMU chapter.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>