nds32: refine nds32_v2 implement

Change-Id: I6e26ffbdcd426a15b34bff022964946f613f318c
Signed-off-by: Hsiangkai Wang <hsiangkai@gmail.com>
Reviewed-on: http://openocd.zylin.com/1569


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