ixp4xx_eth: move platform_data definition

The platform data is needed to compile the driver as standalone,
so move it to a global location along with similar files.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>