xen/acpi-processor: Do not depend on CPU frequency scaling drivers.

With patch "xen/cpufreq: Disable the cpu frequency scaling drivers
from loading." we do not have to worry about said drivers loading
themselves before the xen-acpi-processor driver. Hence we can remove
the default selection (=y if CPU frequency drivers were built-in, or
=m if CPU frequency drivers were built as modules), and just
select =m for the default case.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>