arm64: dts: rockchip: remove static xin32k from px30

Similar to all other Rockchip SoCs the px30 does not have a static
32kHz clock. Instead it again gets supplied from an external component
like the pmic.

So drop the static clock, so that we can hook up the right one.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20190917082659.25549-2-heiko@sntech.de