ACPI: Store SRAT table revision

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.
In order to know whether or not, we must know what version the SRAT
table has.

This patch stores the SRAT table revision for later consumption
by arch specific __init functions.

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