pata: ixp4xx: split platform data to its own header

Portable drivers cannot use mach/platform.h, so move the
structure into its own header. With this, compile testing
can be enabled.

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