This new function, xen_debug_putc(), is intended to be used to
enable CONFIG_DEBUG_UART on xen guest.
Please note that the underlying functionality in Xen is available
only when Xen is configured with !NDEBUG but is much simpler than
a generic HYPERVISOR_console_io().
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>