armv7m: update to use correct register core_cache

The was missed when the armv7m was moved over to using the std arm
core_cache, probably because it is disabled by default.

Change-Id: I2f5a18ef6dd783b36e8c29f4c52379104bda4583
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1138
Tested-by: jenkins