cortex_m.c: Use two byte breakpoint for 32bit Thumb-2 request

When GDB requests a breakpoint on a 32bit Thumb-2 instruction it
sends a length of 3 which the current code rejects. Using the
existing two byte breakpoint for this case appears to work fine.

The use of length==3 for this case is mentioned in a few places:
https://sourceware.org/gdb/onlinedocs/gdb/ARM-Breakpoint-Kinds.html
http://sourceforge.net/p/openocd/mailman/message/30012280/

Change-Id: I59cd69ba4d1bc9a37b86569738c6bb2a67c3eb7a
Signed-off-by: Jon Burgess <jburgess777@gmail.com>
Reviewed-on: http://openocd.zylin.com/2312
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>