docs: platform: Update descriptions for qemu/sifive_u support

With QEMU v4.2 release that has improved the emulation fidelity
of 'sifive_u' machine, OpenSBI v0.4 / U-Boot v2019.10-rc1 / Linux
kernel v5.3-rc2 images built for the SiFive HiFive Unleashed board
can be used out of the box without any special hack.

Update our documents to mention 'qemu/sifive_u' platform should
only be used with QEMU v4.1 or before, and it will be dropped
sometime in the future release.

Going forward, 'sifive/fu540' platform can be used on both real
hardware and QEMU v4.2+ 'sifive_u' machine.

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