dts: bindings: clock: nrf53-hfxo: add startup-time-us prop

Add the clock startup time property to the nrf53 series HFXO binding.

Note that the values added to the soc .dtsi files are worst case
defaults, which will be replaced with optimal values at board level
in the future, as they depend on the specific crystal in use.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>