[CPUFREQ] Move PMBASE reading away and do it only once at initialization time

This patch moves away PMBASE reading and only performs it at
cpufreq_register_driver time by exiting with -ENODEV if unable to read
the value.

Signed-off-by: Mattia Dongili <malattia@linux.it>
Acked-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Dave Jones <davej@redhat.com>