ACPICA: ACPI 6.0: Add ACPI_SUB_PTR().

ACPICA commit 5de82757aef5d6163e37064033aacbce193abbca

Using a minus number with ACPI_ADD_PTR() will cause compiler warnings, such
warnings cannot be eliminated by force casting an unsigned value to a
signed value. This patch thus introduces ACPI_SUB_PTR() to be used with
minus numbers. Lv Zheng.

Link: https://github.com/acpica/acpica/commit/5de82757
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>