ACPICA: Headers: Add GTDT flag definitions for the timer subtable.

Mostly by Hanjun Guo <hanjun.guo@linaro.org>

Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
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>