Add USB code

Adds the usb code that we got in ECE 385. It will not work now, and
parts that involve the timer are disabled. It does compile though, with
a few warnings.

The goal will be to add USB MSD support, which is not actually given to
us so I will have to do myself or find some other code to base it off
of.

sw/gpio.h

@@ -0,0 +1,9 @@
+#ifndef _GPIO_H
+#define _GPIO_H
+
+#include <stdint.h>
+
+#define USB_GPIO (uint8_t*)0x7ff1
+extern uint8_t* usb_gpio;
+
+#endif