powercap/intel_rapl_msr: Convert rapl_msr_priv into pointer

Changes the static struct rapl_msr_priv to a pointer to allow using
a different RAPL MSR interface, preparing for supporting AMD's RAPL
MSR interface.

No functional changes.

Signed-off-by: Victor Ding <victording@google.com>
Acked-by: Kim Phillips <kim.phillips@amd.com>
[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>