Add driver for hex digits

This only includes the method to set the pairs of displays individually.
The functions to write a single 16 or 32 bit value have not been
implemented yet, nor has the mask function.

sw/sevenseg.h

@@ -0,0 +1,12 @@
+#ifndef _SEVEN_SEG
+#define _SEVEN_SEG
+
+#include <stdint.h>
+
+uint8_t hex_set_8(uint8_t val, uint8_t idx);
+uint8_t hex_set_16(uint16_t val);
+uint8_t hex_set_24(uint32_t val);
+
+void hex_enable(uint8_t mask);
+
+#endif