arm64: dts: rockchip: remove 408MHz operating point from px30

It looks like the px30 is running unstable at this 408MHz operating point.
This shows in stalled threads and other big numbers of kernel exception.

At 600MHz and above it instead works stable and as expected. As the 408MHz
point doesn't even decrease the voltage compared to 600MHz, just drop this
408MHz operating point for now.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Link: https://lore.kernel.org/r/20191116095220.31122-1-heiko@sntech.de