ACPI, ia64: Use SRAT table rev to use 8bit or 16/32bit PXM fields (ia64)

In SRAT v1, we had 8bit proximity domain (PXM) fields; SRAT v2 provides
32bits for these. The new fields were reserved before.
According to the ACPI spec, the OS must disregrard reserved fields.

ia64 did handle the PXM fields almost consistently, but depending on
sgi's sn2 platform. This patch leaves the sn2 logic in, but does also
use 16/32 bits for PXM if the SRAT has rev 2 or higher.

The patch also adds __init to the two pxm accessor functions, as they
access __initdata now and are called from an __init function only anyway.

Note that the code only uses 16 bits for the PXM field in the processor
proximity field; the patch does not address this as 16 bits are more than
enough.

Signed-off-by: Kurt Garloff <kurt@garloff.de>
Signed-off-by: Len Brown <len.brown@intel.com>