soc: samd5x: xosc32: Allow users to select the start-up time

**Is your enhancement proposal related to a problem? Please describe.**
The Atmel samd5x have a fixed start-up time for the xosc32. Provide a way to select the start-up time for the xosc32 from samd5x platform.

**Describe alternatives you've considered**
* Kconfig: it is the easy way to implement.
* devicetree: require high effort becuase platform don't provide clock drivers yet.