cpufreq / intel_pstate: Change to disallow module build

Load order is important in order for intel_pstate to take over as the
default scaling driver from acpi-cpufreq.

If both are built-in, acpi-cpufreq uses late_initcall() and
intel_pstate uses device_initcall() so it will be able to register as
the scaling before acpi-cpufreq for the processors supported by
intel_pstate.

If acpi-cpufreq is built as a module then intel_pstate still gets
first option to become the scaling driver.

Signed-off-by: Dirk Brandewie <dirk.j.brandewie@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
cpufreq / intel_pstate: Add kernel command line option disable intel_pstate.

When intel_pstate is configured into the kernel it will become the
preferred scaling driver for processors that it supports.  Allow the
user to override this by adding:
   intel_pstate=disable
on the kernel command line.

Signed-off-by: Dirk Brandewie <dirk.j.brandewie@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>