add interrupt init code (and increase rtc tick rate)

sw/kernel/interrupts/interrupt.h

@@ -0,0 +1,17 @@
+#ifndef _INTERRUPT_H
+#define _INTERRUPT_H
+
+#include <stdint.h>
+
+#define BUTTON  (1 << 0)
+#define UART    (1 << 1)
+
+void irq_int();
+void nmi_int();
+
+void register_irq(void* addr, uint8_t irqn);
+
+uint8_t irq_get_status();
+void irq_set_status(uint8_t);
+
+#endif