HTIF (Host Target Interface) provides console emulation for QEMU. HTIF
allows identical copies of BBL (Berkeley Boot Loader) and linux to run
on both Spike and QEMU. BBL provides HTIF console access via the
SBI (Supervisor Binary Interface) and the linux kernel SBI console.

The HTIT chardev implements the pre qom legacy interface consistent
with the 16550a UART in 'hw/char/serial.c'.
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Signed-off-by: NSagar Karandikar <sagark@eecs.berkeley.edu>
Signed-off-by: NStefan O'Rear <sorear2@gmail.com>
Signed-off-by: NMichael Clark <mjc@sifive.com>