driver core: Fix link to device power management documentation

Correct location as of commit 2728b2d2 (PM / core / docs:
Convert sleep states API document to reST).

Fixes: 2728b2d2

 (PM / core / docs: Convert sleep states API document to reST)
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>