Make IRQ handler weak

Allow the IRQ handler to be overridden for test cases that operate
directly on top of the device driver.

Change-Id: Iddf7aec041a3183583e6e4d8521d5a637c379689
diff --git a/src/ethosu_driver.c b/src/ethosu_driver.c
index cb1790e..d4c4307 100644
--- a/src/ethosu_driver.c
+++ b/src/ethosu_driver.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2019-2020 Arm Limited. All rights reserved.
+ * Copyright (c) 2019-2021 Arm Limited. All rights reserved.
  *
  * SPDX-License-Identifier: Apache-2.0
  *
@@ -201,7 +201,7 @@
  */
 void __attribute__((weak)) ethosu_resume() {}
 
-void ethosu_irq_handler_v2(struct ethosu_driver *drv)
+void __attribute__((weak)) ethosu_irq_handler_v2(struct ethosu_driver *drv)
 {
     uint8_t irq_raised = 0;