ACPICA: ACPI 6.0: Add support for XENV table.

ACPICA commit 08c4197cf4ddd45f0c961078220b0fc19c10745c

Xen Environment table.

Link: https://github.com/acpica/acpica/commit/08c4197c
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>