Add CONFIG_USB_SERIAL and CONFIG_USB_SERIAL_CP210x.

These enable the board to use usb-over-serial to control remote GPIO
pins. The drive-by-wire project uses these to control LEDs in our first
prototype form factor.

Change-Id: I1e39b1deb732a897646e396dcf3496e2a1edf32a
diff --git a/defconfig b/defconfig
index 1249226..29f5651 100644
--- a/defconfig
+++ b/defconfig
@@ -77,4 +77,6 @@
 CONFIG_GOOGLE_APEX_POWER=y
 
 CONFIG_USB_G_HID=y
+CONFIG_USB_SERIAL=y
+CONFIG_USB_SERIAL_CP210X=y