nds32: support hardware prefetcher

We add a config for user to enable or disable this feature.
It can be used to control the hardware prefetch function.

Signed-off-by: Nylon Chen <nylon7@andestech.com>
Acked-by: Greentime Hu <greentime@andestech.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>