ixp4xx_eth: Probe the PTP module from the device tree

This adds device tree probing support for the PTP module
adjacent to the ethernet module. It is pretty straight
forward, all resources are in the device tree as they
come to the platform device.

Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>