powerpc/time: Prepare to stop elapsing in dynticks-idle

Currently the tick subsystem stores the idle cputime accounting in
private fields, allowing cohabitation with architecture idle vtime
accounting. The former is fetched on online CPUs, the latter on offline
CPUs.

For consolidation purpose, architecture vtime accounting will continue
to account the cputime but will make a break when the idle tick is
stopped. The dyntick cputime accounting will then be relayed by the tick
subsystem so that the idle cputime is still seen advancing coherently
even when the tick isn't there to flush the idle vtime.

Prepare for that and introduce three new APIs which will be used in
subsequent patches:

  - vtime_dynticks_start() is deemed to be called when idle enters in
    dyntick mode. The idle cputime that elapsed so far is accumulated.

  - vtime_dynticks_stop() is deemed to be called when idle exits from
    dyntick mode. The vtime entry clocks are fast-forward to current time
    so that idle accounting restarts elapsing from now.

  - vtime_reset() is deemed to be called from dynticks idle IRQ entry to
    fast-forward the clock to current time so that the IRQ time is still
    accounted by vtime while nohz cputime is paused.

Also accumulated vtime won't be flushed from dyntick-idle ticks to avoid
accounting twice the idle cputime, along with nohz accounting.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Link: https://patch.msgid.link/20260508131647.43868-6-frederic@kernel.org

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