intel_rapl: cleanup some functions

Previously, there are three functions:
rapl_register_psys(), which registers platform rapl domain.
rapl_register_powercap(), which registers powercap control type.
rapl_unregsiter_powercap(), which unregisters platform rapl domain and
powercap control type.

This is confusing as the function name does not describe what it does
clearly.

With this patch, the three functions are removed, and two new functions
rapl_register_platform_domain()/rapl_unregister_platform_domain() are
introduced instead, and they do exactly what their function name describes.

Plus, as part of the common code, hardcoded MSR accesses in these functions
are converted to follow the abstracted register access.

Reviewed-by: Pandruvada, Srinivas <srinivas.pandruvada@intel.com>
Tested-by: Pandruvada, Srinivas <srinivas.pandruvada@intel.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>