x86 ACPI sleep: Fix a bug when ACPI registers in System_Memory

x86 ACPI sleep: Fix a bug when ACPI registers in System_Memory

Some ACPI registers may be in System_Memory but not System_IO space,
For these machines S3 fails since current Xen use I/O instructions (e.g.
inb(), outb(), etc.) to access ACPI registers. The patch attached fix
the bug by adding GAS (generic address structure) support ported from
Linux, which should also benefit later ACPI activity within Xen.

Signed-off-by: Huacai Chen <huacai.chen@intel.com>