linker: add __isr tag

This allows interrupt service routine to be declared with
the __isr tag so they can be placed in the correct linker
section. The first would be putting them into the pinned
section.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>
diff --git a/include/linker/section_tags.h b/include/linker/section_tags.h
index eaace84..39549c8 100644
--- a/include/linker/section_tags.h
+++ b/include/linker/section_tags.h
@@ -80,6 +80,12 @@
 #define __pinned_noinit	__noinit
 #endif /* CONFIG_LINKER_USE_PINNED_SECTION */
 
+#if defined(CONFIG_LINKER_USE_PINNED_SECTION)
+#define __isr		__pinned_func
+#else
+#define __isr
+#endif
+
 #endif /* !_ASMLANGUAGE */
 
 #endif /* ZEPHYR_INCLUDE_LINKER_SECTION_TAGS_H_ */