timekeeping: Add clocksource ID to struct system_counterval_t

Clocksource pointers can be problematic to obtain for drivers which are not
clocksource drivers themselves. In particular, the RFC virtio_rtc driver
[1] would require a new helper function to obtain a pointer to the ARM
Generic Timer clocksource. The ptp_kvm driver also required a similar
workaround.

Add a clocksource ID member to struct system_counterval_t, which in the
future shall identify the clocksource, and which shall replace the struct
clocksource * member. By this, get_device_system_crosststamp() callers
(such as virtio_rtc and ptp_kvm) will be able to supply easily accessible
clocksource ids instead of clocksource pointers.

[1] https://lore.kernel.org/lkml/20231218073849.35294-1-peter.hilber@opensynergy.com/

Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240201010453.2212371-3-peter.hilber@opensynergy.com