PM: domains: replace -ENOTSUPP with -EOPNOTSUPP

While submitting a patch to add next_wakeup, checkpatch reported this -

WARNING: ENOTSUPP is not a SUSV4 error code, prefer EOPNOTSUPP
+       return -ENOTSUPP;

Address the above warning in other functions in pm_domain.h.

Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Lina Iyer <ilina@codeaurora.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>