ACPI: DPTF: Add ACPI_DPTF Kconfig menu

Add a Kconfig menu for Intel DPTF (Dynamic Platform and Thermal
Framework), put both the existing participant drivers in it and set
them to be built as modules by default.

While at it, do a few assorted cleanups for a good measure.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Acked-by: Borislav Petkov <bp@suse.de>