[coreboot] amdk8_util.asl fixes, documentation, and ACPI question

[Myles Watson]

This patch mostly documents the functionality of amdk8_util.asl.  It
also fixes a couple of logical errors that were corrupting the
resources returned from _CRS.

Fixes:
1. When adding a MMIO region to the southbridge, the previous region
was corrupted.
        - Save previous region before checking if you're handling the
southbridge.
        - Make sure you're on node 0 before adding the TOM region.
        - Add in a MMAX value of 0xFFDFFFFF (See the ACPI question at the end.)
        - Store resulting region with the saved region.
        - Move increment of Local 4 so it's more obvious what it refers to.
    Before: [TOM - 0xBFFFF] added to every node with Link 0 enabled
    After: [0xA0000-0xBFFFF] and [TOM-0xFFDFFFFF] only once
2. Missing RTAG call corrupted return value when there was no resource
for the link
        - Add RTAG (BUF0)

Signed-off-by: Myles Watson

But the question is:

Why is this logic here?  Why do you want a region from TOM-(4G-2M)*
when you explicitly have all the regions that are reachable from the
HT links in other resources?

* I got 4G-2M from the ACPI FAQ.

Thanks,
Myles