platform/x86: ideapad-laptop: Expose conservation mode switch

This exposes the battery conservation mode present on some (?) IdeaPads.
The mode is set by calling ACPI method SBMC with argument 3 (on) or
5 (off). Status is reported in bit 5 of the return value of ACPI method
GBMD.

Signed-off-by: Hao Wei Tee <angelsl@in04.sg>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>