For no good reason I had assumed that ACPI table headers would be page

aligned (or at least not cross a page boundary).  However, it turns out
that on at least one machine one table header does cross a page boundary.
This caused problems with the MADT early probe as it uses the crash dump
map to load ACPI tables by loading the RSDT/XSDT into pages 1 ... N and
loading the header of each ACPI table header into page 0 looking for the
MADT.  However, if a table header crossed a page boundary, then page 1
would get trashed resulting in a panic.  Fix this by reserving the first
2 pages for ACPI table headers (headers are less than a page in size,
so 2 pages will be sufficient) and use pages 2 .. N for the RSDT and XSDT.

Note: amd64 should probably be simplified to just use pmap_mapbios()
for all these tables which will use the direct map and not need the
crash dump hack.

MFC after:	5 days
Tested on:	i386
Reported by:	Pete French  petefrench of ticketswitch.com

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