hwrng: timeriomem - Breaks an allyesconfig build on s390:

  CC      drivers/char/hw_random/timeriomem-rng.o
drivers/char/hw_random/timeriomem-rng.c: In function 'timeriomem_rng_data_read':
drivers/char/hw_random/timeriomem-rng.c:60: error: implicit declaration of function 'readl'

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>