riscv/virt: Jump to pflash if specified

riscv/virt: Jump to pflash if specified

If the user supplied pflash to QEMU then change the reset code to jump
to the pflash base address instead of the DRAM base address.

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