Xen: ACPI: Hide UART used by Xen

ACPI 6.0 introduces a new table STAO to list the devices which are used
by Xen and can't be used by Dom0. On Xen virtual platforms, the physical
UART is used by Xen. So here it hides UART from Dom0.

CC: "Rafael J. Wysocki" <rjw@rjwysocki.net> (supporter:ACPI)
CC: Len Brown <lenb@kernel.org> (supporter:ACPI)
CC: linux-acpi@vger.kernel.org (open list:ACPI)
Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>