cortex_m: Discover the AP to use, just like Cortex-A

cortex_m: Discover the AP to use, just like Cortex-A

This required fixing the AP ID parsing in dap_find_ap() to
match IHI0031C. The AXI type was added too.

Change-Id: I44577a7848df37586e650dce0fb57ac26f5f858c
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/3146
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>