target: cortex_a: fix segfault when SPSR is not properly handled

OpenOCD doesn't (yet) know how to handle HYP mode properly so spsr
register is not getting initialised when OpenOCD connects to a target
stopped in this mode.

Reported on IRC by thinkfat and nearffxx.

Change-Id: I4bda9ba0c582c8e9cacefe708cc4a3d947151f84
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/3906


Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>
Reviewed-by: Chengyu Zheng <chengyu.zheng@polimi.it>