vendorcode/amd/agesa/f15tn: Fix erratum #712

Implement the fix for the erratum #712. - Processor May Hang During Graphics Memory Controller
Sequencing

The processor may hang during a graphics memory controller (GMC) sleep state transitioning. The failure may
be processor specific and may be sensitive to temperature.

Potential Effect on System:
System hang.

Suggested Workaround:
BIOS should set D18F2x408_dct[1:0] bit 31 = 1b.

See Publication # 48931 Revision: 3.08

Change-Id: I4346fd4ef3cf554ffdaaad5ab6fc84e73532e885
Signed-off-by: Rudolf Marek <r.marek@assembler.cz>
Reviewed-on: http://review.coreboot.org/6216
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <gaumless@gmail.com>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>