MLBEDSW-1796 Refactor IRQ initialization

Turn the IRQ initialization around, to have the application define the
interrupt handler and expose the driver routine that is to be called
when that happens.

Change-Id: Idbfba1b1d1a1eaf6678ee16e9583c496eb2287ed
diff --git a/include/ethosu_driver.h b/include/ethosu_driver.h
index 5d75852..7ab420a 100644
--- a/include/ethosu_driver.h
+++ b/include/ethosu_driver.h
@@ -92,6 +92,11 @@
  */
 void ethosu_abort(void);
 
+/**
+ * Interrupt handler do be called on IRQ from Ethos-U
+ */
+void ethosu_irq_handler(void);
+
 #ifdef __cplusplus
 }
 #endif