x86, hpet: Add reference to chipset erratum documentation for disable-hpet-msi-quirk

(At the moment the "SB700 Family Product Errata" document is available
at http://support.amd.com/us/Embedded_TechDocs/46837.pdf

)

Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
LKML-Reference: <20100517164324.GB10254@alberich.amd.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>