Cortex A/R : Allow interrupt disable during single-step

Example usage:
	cortex_a maskisr on
	cortex_a maskisr off
	cortex_r maskisr on
	cortex_r maskisr off

Change-Id: I799288d9b848a06f561ba29ec1eb8e5eeace5685
Signed-off-by: Evan Hunter <ehunter@broadcom.com>
Reviewed-on: http://openocd.zylin.com/2876
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>

src/target/cortex_a.h

@@ -55,6 +55,11 @@
 
 #define CORTEX_A_PADDRDBG_CPU_SHIFT 13
 
+enum cortex_a_isrmasking_mode {
+	CORTEX_A_ISRMASK_OFF,
+	CORTEX_A_ISRMASK_ON,
+};
+
 struct cortex_a_brp {
 	int used;
 	int type;
@@ -91,6 +96,8 @@ struct cortex_a_common {
 	uint32_t ttypr;
 	uint32_t didr;
 
+	enum cortex_a_isrmasking_mode isrmasking_mode;
+
 	struct armv7a_common armv7a_common;
 
 };