s390/pai_crypto: Add common pai_read() function

s390/pai_crypto: Add common pai_read() 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_read() to read counter values.
The function expects a PAI PMU specific read function as second
parameter.

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>