ARM: kirkwood: Remove all remaining trace of DNS-320/325 platform code

No platform code left now, so remove all references to it.

The reset gpio code didn't work to begin with, it will be replaced with
a DT node at a future date. -jc

Signed-off-by: Jamie Lentin <jm@lentin.co.uk>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>