nds32: always polling after gdb attached

Do not turn on/off polling as leave/enter debug mode.
Enable polling after gdb attached, and disable polling
after gdb detached.

Change-Id: Id64459b86f44937af7ea5ccfe2cd13e31732eecf
Signed-off-by: Hsiangkai Wang <hsiangkai@gmail.com>
Reviewed-on: http://openocd.zylin.com/1574


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>