s390/crypto: use TRNG for seeding/reseeding

With the z14 machine there came also a CPACF hardware extension
which provides a True Random Number Generator. This TRNG can
be accessed with a new subfunction code within the CPACF prno
instruction and provides random data with very high entropy.

So if there is a TRNG available, let's use it for initial seeding
and reseeding instead of the current implementation which tries
to generate entropy based on stckf (store clock fast) jitters.

For details about the amount of data needed and pulled for
seeding and reseeding there can be explaining comments in the
code found.

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