s390/cpum_cf: introduce kernel_cpumcf_avail() function

A preparation to move out common CPU-MF counter facility support
functions, first introduce a function that indicates whether the
support is ready to use.

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