mainboard/asus/kgpe-d16: Limit HT speed to 2.6GHz

The CPU <--> CPU HT wiring on this board has only been validated
to 2.6GHz.  While higher frequencies appear to function initially,
and in fact function when only one CPU package is installed, dual
CPU package systems will lock up after around 6 - 12 hours of uptime
due to presumed HT link errors at the higher (>= 2.8GHz) HT clocks.

If applications are not being used that stress the coherent fabric,
then the uptime before hang may be much longer.  Users attempting
to overclock the HT links are advised to "burn in test" the HT links
by running memtester locked to a node with no local memory installed.

Change-Id: I8fae90c67aa0e8b103e9b8906dea50d1e92ea5a9
Signed-off-by: Timothy Pearson <tpearson@raptorengineeringinc.com>
Reviewed-on: https://review.coreboot.org/12064
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/mainboard/asus/kgpe-d16/romstage.c

@@ -322,6 +322,11 @@ void cache_as_ram_main(unsigned long bist, unsigned long cpu_init_detectedx)
 
 	struct sys_info *sysinfo = &sysinfo_car;
 
+	/* Limit the maximum HT speed to 2.6GHz to prevent lockups
+	 * due to HT CPU <--> CPU wiring not being validated to 3.2GHz
+	 */
+	sysinfo->ht_link_cfg.ht_speed_limit = 2600;
+
 	uint32_t bsp_apicid = 0, val;
 	uint8_t byte;
 	msr_t msr;