Add dsp

With:
- LED control
- UART
- PDM out
- square wave generator (DMA to PDM out (was it really necessary?))
- sample program that plays a square wave from UART values

dsp/sys.h

@@ -0,0 +1,15 @@
+#pragma once
+
+#include "led.h"
+#include "uart.h"
+#include "wave.h"
+
+#define LED0_BASE    0xc000
+#define UART0_BASE   0xc010
+#define PDMOUT0_BASE 0xc020
+#define SQUARE0_BASE 0xc030
+
+#define led0 ((struct led*) LED0_BASE)
+#define uart0 ((struct uart*) UART0_BASE)
+#define pdmout0 ((struct pdmout*) PDMOUT0_BASE)
+#define square0 ((struct square*) SQUARE0_BASE)