libpayload: disable EHCI & XHCI in defconfig-mips

drivers/usb/xhci.c and drivers/usb/ehci.c both require arch/barrier.h.

barrier.h is present for x86, arm, and arm64, but not for mips. This
is generating a build error after enabling USB by default on libpayload.

I believe that this slipped through the buiders due to them not getting
cleaned fully.  It was caught in the coverity scan and when setting up
a new build server.

Change-Id: Idd89409a048009c087ce2a787d96a1efd089157f
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/14391
Tested-by: build bot (Jenkins)
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
diff --git a/payloads/libpayload/configs/defconfig-mips b/payloads/libpayload/configs/defconfig-mips
index 7e31478..4a0a914 100644
--- a/payloads/libpayload/configs/defconfig-mips
+++ b/payloads/libpayload/configs/defconfig-mips
@@ -2,3 +2,5 @@
 CONFIG_LP_COREBOOT_VIDEO_CONSOLE=y
 CONFIG_LP_PC_KEYBOARD=y
 CONFIG_LP_TIMER_IMG_PISTACHIO=y
+# CONFIG_LP_USB_EHCI is not set
+# CONFIG_LP_USB_XHCI is not set