x86/ACPI: CPPC: Rename cppc_msr.c to cppc.c

Rename the cppc_msr.c to cppc.c in x86 ACPI, that expects to use this file
to cover more function implementation for ACPI CPPC beside MSR helpers.
Naming as "cppc" is more straightforward as one of the functionalities
under ACPI subsystem.

Signed-off-by: Huang Rui <ray.huang@amd.com>
[ rjw: Subject ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>