cpufreq: intel_pstate: Enable HWP during system resume on CPU0

When maxcpus=1 is in the kernel command line, the BP is responsible
for re-enabling the HWP - because currently only the APs invoke
intel_pstate_hwp_enable() during their online process - which might
put the system into unstable state after resume.

Fix this by enabling the HWP explicitly on BP during resume.

Reported-by: Doug Smythies <dsmythies@telus.net>
Suggested-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Yu Chen <yu.c.chen@intel.com>
[ rjw: Subject/changelog, minor modifications ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>