hw/riscv: spike: Change the default bios to use generic platform image

To keep sync with other RISC-V machines, change the default bios to
use generic platform fw_dynamic.elf image.

While we are here, add some comments to mention that using ELF files
for the Spike machine was intentional.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <1596439832-29238-6-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>