s390/pai_crypto: Add common pai_start() function

s390/pai_crypto: Add common pai_start() function

To support one common PAI PMU device driver which handles
both PMUs pai_crypto and pai_ext, use a common naming scheme
for structures and variables suitable for both device drivers.

Add a common usable function pai_start() to the event on a CPU.
The function expects a PAI PMU specific read function as second
parameter to read out the start value for an event.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Jan Polensky <japo@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>