riscv: qemu: Enable HTIF console support

Enable support for HTIF console so that we can use QEMU RISC-V U-Boot
on RISC-V emulators and ISS having it.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>