cortex_a: fix reset for SWD transport

Change b0698501 fixed
reset for i.MX6 and TI Sitara SoCs but broke reset for
cortex-a targets that use SWD. This patch is a work-
around that forces asserting SRST when SWD is used.

Change-Id: I7e39f2a469b9b4b2b74ad48ba49f2eeb58528921
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/3641


Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>