ixp4xx_eth: Add devicetree bindings

This adds device tree bindings for the IXP46x PTP Timer, a companion
to the IXP4xx ethernet in newer platforms.

Cc: devicetree@vger.kernel.org
Cc: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>