wtm2: fix build breakage and misconfiguration

In some header shuffling stdint.h no longer included a definition
for NULL. Pull in string.h for the proper definition. Also, disable
the xHCI controller in libpayload as the firmware leaves control
of the USB 3.0 ports to the EHCI controller.

BUG=None
BRANCH=None
TEST=Was able to boot in in non-dev mode. In dev mode there were no
     longer errors about the xHCI controller.

Change-Id: Iabf15b3b17d88784e0718dc9f0fd885e6551e0b1
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/60874
Reviewed-by: Sameer Nanda <snanda@chromium.org>

src/mainboard/intel/wtm2/romstage.c

@@ -19,6 +19,7 @@
  */
 
 #include <stdint.h>
+#include <string.h>
 #include <console/console.h>
 #include "cpu/intel/haswell/haswell.h"
 #include "northbridge/intel/haswell/haswell.h"