Add UART interrupts

Currently an interrupt is triggered any time there is any activity on
the UART_RXD line, but later it will only trigger once there is data
ready to be read.

sw/interrupt.h

@@ -4,6 +4,7 @@
 #include <stdint.h>
 
 #define BUTTON  (1 << 0)
+#define UART    (1 << 1)
 
 void irq_int();
 void nmi_int();

sw/irq.c

@@ -17,4 +17,8 @@ void handle_irq() {
         puts("Button Interrupt!\n");
         irq_set_status(status & ~BUTTON);
     }
+    if (status & UART) {
+        puts("UART Interrupt!\n");
+        irq_set_status(status & ~UART);
+    }
 }