ACPI / PM: Register acpi_power_driver early

The ACPI device driver used for handling power resources,
acpi_power_driver, creates a struct acpi_power_resource object for
each ACPI device representing a power resource.  These objects are
then used when setting and reading the power states of devices using
the corresponding power resources.  Unfortunately, acpi_power_driver
is registered after acpi_scan_init() that may add devices using the
power resources before acpi_power_driver has a chance to create
struct acpi_power_resource objects for them (specifically, the power
resources may be referred to during the scanning process through
acpi_bus_get_power() before they have been initialized).

As the first step towards fixing this issue, move the registration
of acpi_power_driver into acpi_scan_init() so that power resource
devices can be initialized by it as soon as they have been found in
the namespace.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Len Brown <len.brown@intel.com>