ARM: ixp4xx: enable multiplatform support

After all the work that Linus Walleij did on this platform, it can be
part of a generic kernel build as well.

Note that there are known bugs in little-endian mode on ixp4xx, and
no other ARMv5 platform at this point supports big-endian mode, or is
likely to in the future, so there is limited practical value in this,
but it helps with build testing and ixp4xx little-endian support may
get fixed in the future.

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