Add a weak function to process HWP (Hardware P-states) notifications and
move updating HWP_STATUS MSR to this function.

This allows HWP interrupts to be processed by the intel_pstate driver in
HWP mode by overriding the implementation.
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>