Add SD card software interface

Adds a function to send a command to the sd card.

sw/sd_card.h

@@ -0,0 +1,8 @@
+#ifndef _SD_CARD_H
+#define _SD_CARD_H
+
+#include <stdint.h>
+
+void sd_card_command(uint32_t arg, uint8_t cmd);
+
+#endif