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/main.c

@@ -0,0 +1,35 @@
+#include "sys.h"
+
+void init() {
+  square0->output_address = pdmout0;
+  square0->high_val = 0xffff;
+  square0->low_val = 0x0000;
+  square0->flags = SQUARE_FLAG_ENABLE_msk;
+  // will set the period later
+}
+
+int main() {
+  init();
+
+  while(1) {
+    uint8_t c = uart_read(uart0);
+    uint16_t period;
+    if (c == 'c') {
+      period = 1493;  // C4, 262.63 Hz, maybe
+    } else if (c == 'd') {
+      period = 1330;  // D4, 293.66 Hz, maybe
+    } else if (c == 'e') {
+      period = 1185;  // E4, 329.63 Hz, maybe
+    } else if (c == 'f') {
+      period = 1119;  // F4, 249.23 Hz, maybe
+    } else if (c == 'g') {
+      period = 996;  //  G4, 392.00 Hz, maybe
+    } else if (c == 'a') {
+      period = 887;  //  A4, 440.00 Hz, maybe
+    } else if (c == 'b') {
+      period = 791;  //  B4, 493.88 Hz, maybe
+    }
+
+    square0->period = period;
+  }
+}