arm64: dts: rockchip: Correct PMU compatibles for PX30 and RK3308

A proper binding for the Cortex-A35 PMU actually predates these DTs
being upstreamed, so use it.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/6dfed94a99780c2314b38ff2b55a7efa0be4edbc.1587125314.git.robin.murphy@arm.com


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