include: clock_control: Fix int return value

[alexanderwachter]

The return value of clock_control_get_status is a clock_control_status
enum, but in case the device is not ready, an int is returned.
This leads to compiler errors. Introduced a new enum value
CLOCK_CONTROL_STATUS_UNAVAILABLE and return it instead of -ENODEV.

Fixes #32385

[alexanderwachter]

Alternative:

enum clock_control_status {
	CLOCK_CONTROL_STATUS_STARTING = 0,
	CLOCK_CONTROL_STATUS_OFF = 1,
	CLOCK_CONTROL_STATUS_ON = 2,
	CLOCK_CONTROL_STATUS_UNKNOWN	= 3,
	CLOCK_CONTROL_STATUS_UNAVAILABLE = -ENODEV,
};