s390/trng: Introduce s390 TRNG device driver.

This patch introduces a new device driver s390-trng for the
s390 platform which exploits the new PRNO TRNG cpacf
subfunction. The true-random-number-generator is accessible
from userspace, by default visible as /dev/trng. The driver
also registers at the kernel build-in hwrng API to feed the
hwrng with fresh entropy data. This generic device driver
for hardware random data is visible from userspace as
/dev/hwrng.

Signed-off-by: Harald Freudenberger <freude@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>