hw_random: ixp4xx: enable compile-testing

The driver is almost portable already, it just needs to
include the new header for the cpu definition.

Cc: Deepak Saxena <dsaxena@plexity.net>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>