cpu/amd/model_fxx: Clear out unused / stale MTRRs in ramstage

This mirrors a similar commit made to Family 10h support
in changeset 11966 file model_10xxx_init.c

TEST: Booted ASS KFSN4-DRE with 1x Opteron 8222

Change-Id: I760ef27be00aed11c0ac21b9bd741189f4b05834
Signed-off-by: Timothy Pearson <tpearson@raptorengineeringinc.com>
Reviewed-on: http://review.coreboot.org/12250
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/cpu/amd/model_fxx/model_fxx_init.c

@@ -460,6 +460,18 @@ static void model_fxx_init(device_t dev)
 
 	/* Turn on caching if we haven't already */
 	x86_enable_cache();
+
+	/* Initialize all variable MTRRs except the first pair */
+	msr.hi = 0x00000000;
+	msr.lo = 0x00000000;
+
+	disable_cache();
+
+	for (i = 0x2; i < 0x10; i++) {
+		wrmsr(0x00000200 | i, msr);
+	}
+
+	enable_cache();
 	amd_setup_mtrrs();
 	x86_mtrr_check();