pmdomain: core: Support naming idle states

Commit 422f2d41 ("arm64: dts: qcom: Drop undocumented domain
"idle-state-name"") brought to light the common misbelief that
idle-state-names also applies to e.g. PSCI power domain idle states.

Make that a reality, mimicking the property name used by cpuidle
states.

Signed-off-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Message-ID: <20241130-topic-idle_state_name-v1-2-d0ff67b0c8e9@oss.qualcomm.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>