Updates for timekeeping and PTP core: The cross-timestamp mechanism which allows to correlate hardware clocks uses clocksource pointers for describing the correlation. That's suboptimal as drivers need to obtain the pointer, which requires needless exports and exposing internals. This can be completely avoided by assigning clocksource IDs and using them for describing the correlated clock source. This update adds clocksource IDs to all clocksources in the tree which can be exposed to this mechanism and removes the pointer and now needless exports. This is separate from the timer core changes as it was provided to the PTP folks to build further changes on top. A related improvement for the core and the correlation handling has not made it this time, but is expected to get ready for the next round.