Add is_locked status to clock_control_driver_api

[ofirshe]

Hey,

Many PLL drivers expose the clock_control_driver_api, and I believe it's essential to support a status indicating whether the PLL is locked. This is a critical attribute for the normal operation of a PLL. While it could be encapsulated using the sensor_driver_api's channel_get function with a custom attribute, it seems more appropriate to integrate this functionality directly into the clock_control_driver_api.