time: Introduce one suspend clocksource to compensate the suspend time

On some hardware with multiple clocksources, we have coarse grained
clocksources that support the CLOCK_SOURCE_SUSPEND_NONSTOP flag, but
which are less than ideal for timekeeping whereas other clocksources
can be better candidates but halt on suspend.

Currently, the timekeeping core only supports timing suspend using
CLOCK_SOURCE_SUSPEND_NONSTOP clocksources if that clocksource is the
current clocksource for timekeeping.

As a result, some architectures try to implement read_persistent_clock64()
using those non-stop clocksources, but isn't really ideal, which will
introduce more duplicate code. To fix this, provide logic to allow a
registered SUSPEND_NONSTOP clocksource, which isn't the current
clocksource, to be used to calculate the suspend time.

Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Miroslav Lichvar <[email protected]>
Cc: Richard Cochran <[email protected]>
Cc: Prarit Bhargava <[email protected]>
Cc: Stephen Boyd <[email protected]>
Cc: Daniel Lezcano <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>
Reviewed-by: Daniel Lezcano <[email protected]>
Suggested-by: Thomas Gleixner <[email protected]>
Signed-off-by: Baolin Wang <[email protected]>
[jstultz: minor tweaks to merge with previous resume changes]
Signed-off-by: John Stultz <[email protected]>

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