All IXP4xx platforms are converted to device tree, the platform
data path is no longer used. Drop the code and custom include,
confine the driver in its own file.

Depend on OF and remove ifdefs around this, as we are all probing
from OF now.

Cc: David S. Miller <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: netdev@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20220211223238.648934-9-linus.walleij@linaro.orgSigned-off-by: Linus Walleij <linus.walleij@linaro.org>