hw/riscv: spike: Allow using binary firmware as bios

hw/riscv: spike: Allow using binary firmware as bios

Currently, we have to use OpenSBI firmware ELF as bios for the spike
machine because the HTIF console requires ELF for parsing "fromhost"
and "tohost" symbols.

The latest OpenSBI can now optionally pick-up HTIF register address
from HTIF DT node so using this feature spike machine can now use
OpenSBI firmware BIN as bios.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>