Add basic UART driver

This didn't have a chance to get tested so I hope it works.

There is not fancy stuff here, just write to the register and wait till
it is done.

sw/uart.h

@@ -0,0 +1,11 @@
+#ifndef _UART_H
+#define _UART_H
+
+#include <stdint.h>
+
+void uart_txb(uint8_t val);
+void uart_txb_block(uint8_t val);
+
+uint8_t uart_status();
+
+#endif