PM / Domains: Allow no domain-idle-states DT property in genpd when parsing

Commit 2c361684 ("PM / Domains: Don't treat zero found compatible idle
states as an error"), moved of_genpd_parse_idle_states() towards allowing
none compatible idle state to be found for the device node, rather than
returning an error code.

However, it didn't consider that the "domain-idle-states" DT property may
be missing as it's optional, which makes of_count_phandle_with_args() to
return -ENOENT. Let's fix this to make the behaviour consistent.

Fixes: 2c361684 ("PM / Domains: Don't treat zero found compatible idle states as an error")
Reported-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Cc: 4.20+ <stable@vger.kernel.org> # 4.20+
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/base/power/domain.c

@@ -2653,7 +2653,7 @@ static int genpd_iterate_idle_states(struct device_node *dn,
 
 	ret = of_count_phandle_with_args(dn, "domain-idle-states", NULL);
 	if (ret <= 0)
-		return ret;
+		return ret == -ENOENT ? 0 : ret;
 
 	/* Loop over the phandles until all the requested entry is found */
 	of_for_each_phandle(&it, ret, dn, "domain-idle-states", NULL, 0) {