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>