ACPICA: Tables: Cleanup RSDP signature codes.

This patch introduces new macors to handle RSDP signature and cleans up the
affected codes.  Lv Zheng.
Some updates are only used for ACPICA utilities which are not shipped in
the kernel yet.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>