Make the Atmel SAM3 family SWD-aware

Atmel's SAM3 and SAM4 processor families are very close to each other
in many respects.  However, so far, only the SAM4 target script
contained the magic to allow using SWD, while SAM3 was tied to JTAG
only.  This e.g. prevented the CMSIS-DAP driver from accessing SAM3
devices as it only uses SWD transport (by now).

The patch pulls all the things from the SAM4 target script that are
also applicable to SAM3 devices.  With the patch, an Atmel CMSIS-DAP
debugger (Atmel-ICE) was proven to be able to successfully attach to a
SAM3S-EK evaluation kit.  I also cross-checked that accessing through
a SAM-ICE (Segger J-Link) still works with the patch.

Change-Id: I20dafbff8e1e9f967da950e48a56205586eeef8d
Signed-off-by: Jörg Wunsch <openocd@uriah.heep.sax.de>
Reviewed-on: http://openocd.zylin.com/2046
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>