riscv/virt: Add the PFlash CFI01 device

riscv/virt: Add the PFlash CFI01 device

Add the CFI01 PFlash to the RISC-V virt board. This is the same PFlash
from the ARM Virt board and the implementation is based on the ARM Virt
board. This allows users to specify flash files from the command line.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>