serial: Add RISC-V HTIF console driver

Quite a few RISC-V emulators and ISS (including Spike) have host
transfer interface (HTIF) based console. This patch adds HTIF
based console driver for RISC-V platforms which depends totally
on DT node for HTIF register base address.

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