PM / Domains: Document cpuidle-related functions and change their names

The names of the cpuidle-related functions in
drivers/base/power/domain.c are inconsistent with the names of the
other exported functions in that file (the "pm_" prefix is missing
from them) and they are missing kerneldoc comments.

Fix that by adding the missing "pm_" prefix to the names of those
functions and add kerneldoc comments documenting them.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>