ACPI / PMIC: Stop xpower OPRegion handler relying on IIO

The intel_pmic_xpower code provides an OPRegion handler, which must be
available before other drivers using it are loaded, which can only be
ensured if both the mfd and opregion drivers are built in, which is why
the Kconfig option for intel_pmic_xpower is a bool.

The use of IIO is causing trouble for generic distro configs here as
distros will typically want to build IIO drivers as modules and there
really is no reason to use IIO here. The reading of the ADC value is a
single regmap_bulk_read, which is already protected against races by
the regmap-lock.

This commit removes the use of IIO, allowing distros to enable the
driver without needing to built IIO in and also actually simplifies
the code.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>