s390/cpu_mf: replace stcctm5() with the stcctm() function

Remove the stcctm5() function to extract counters from the MT-diagnostic
counter set with the stcctm() function.  For readability, introduce an
enum to map the counter sets names to respective numbers for the stcctm
instruction.

Signed-off-by: Hendrik Brueckner <brueckner@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>