ACPI / PMIC: Move TPS68470 OpRegion driver to drivers/acpi/pmic/

It is revealed now that TPS68470 OpRegion driver has been added
in slightly different scope. Let's move it to the drivers/acpi/pmic/
folder for sake of the unification.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>