arm64: dts: rockchip: add pd_npu label for RK3588 power domains

arm64: dts: rockchip: add pd_npu label for RK3588 power domains

The NPU of the RK3588 has an external supply. This supply also affects
the power domain of the NPU, not just the NPU device nodes themselves.
Since correctly modelled boards will want the power domain to be aware
of the regulator so that it doesn't always have to be on, add a label to
the NPU power domain node so board files can reference it.

Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Tomeu Vizoso <tomeu@tomeuvizoso.net>
Link: https://lore.kernel.org/r/20250721-6-10-rocket-v9-7-77ebd484941e@tomeuvizoso.net
Signed-off-by: Heiko Stuebner <heiko@sntech.de>