arm64: dts: rockchip: Add RK3328 idle state

Downstream RK3328 DTBs describe a CPU idle state matching that present
on other SoCs like RK3399. This works with upstream Trusted Firmware-A
too, so let's add it here.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/a8c83e705d387446ea8121516d410e38b2d9c57b.1577640736.git.robin.murphy@arm.com


Signed-off-by: Heiko Stuebner <heiko@sntech.de>