cortex_m: set the debug reason to DBGRQ when NVIC_DFSR indicates EXTERNAL

cortex_m: set the debug reason to DBGRQ when NVIC_DFSR indicates EXTERNAL

By definition the EXTERNAL bit in Debug Fault Status Register indicates
that an external debug request (EDBGRQ) signal was asserted.

Usage example: this could be done by CTI in multicore devices in order to
halt all the cores together.

Change-Id: I7830455ce5da6702b7d08c8fa7bfe80e4d8a5055
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/5157
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>