ARM: ixp4xx: Turn the NPE into a platform device

Instead of registering everything related to the NPE
unconditionally in the module_init() call (which will
never work with multiplatform) create a platform device
and probe the NPE like any other device.

Put the device first in the list of devices added for
the platform so it is there when the dependent network
and crypto drivers probe later on.

This probe() path will not be taken unconditionally on
device tree boots, so remove the DT guard.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>