ACPICA: Back port and refine validation of the XSDT root table.

Some platforms contain an XSDT that is ill-formed or otherwise invalid
(such as containing some or all entries that are NULL pointers).
This change adds a new function to validate the XSDT before actually
using it. If the XSDT is found to be invalid, ACPICA will now fall
back to using the RSDT instead.

This feature is already in the Linux kernel.  When it is back ported to
ACPICA, code is refined to follow ACPICA coding style and this patch
is the generation of the integration.

Original-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>