x86/AMD: apply workaround for AMD F16h erratum 792

x86/AMD: apply workaround for AMD F16h erratum 792

Workaround for the Erratum will be in BIOSes spun only after
Jan 2014 onwards. But initial production parts shipped in 2013
itself. Since there is a coverage hole, we should carry this fix
in software in case BIOS does not do the right thing or someone
is using old BIOS.

Description:
 Processor does not ensure DRAM scrub read/write sequence is atomic wrt
 accesses to CC6 save state area. Therefore if a concurrent scrub
 read/write access is to same address the entry may appear as if it is
 not written. This quirk applies to Fam16h models 00h-0Fh

See "Revision Guide" for AMD F16h models 00h-0fh, document 51810 rev.
3.04, Nov 2013.

Equivalent Linux patch link:
 http://marc.info/?l=linux-kernel&m=139066012217149&w=2

Tested the patch on Fam16h server platform and it works fine.

Signed-off-by: Aravind Gopalakrishnan <Aravind.Gopalakrishnan@amd.com>
Reviewed-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Corrected checking for boot CPU. Made warning message conditional.
Compacted warning message text. Moved comment to commit message.

Signed-off-by: Jan Beulich <jbeulich@suse.com>