ACPI / PM: Register power resource devices as soon as they are needed

Depending on the organization of the ACPI namespace, power resource
device objects may generally be scanned after the "regular" device
objects that they are referred from through _PRn.  This, in turn, may
cause acpi_bus_get_power_flags() to attempt to access them through
acpi_bus_init_power() before they are registered (and initialized by
acpi_power_driver).  [This is not a theoretical issue, it actually
happens for one PnP device on my testbed HP nx6325.]

To fix this problem, make acpi_bus_get_power_flags() attempt to
register power resource devices as soon as they have been found in
the _PRn output for any other devices.

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