When a CPU is shutdown its architected timer comparators registers are
lost. Within CPU idle, before processors enter shutdown they enter
clock events broadcast mode through the

clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_ENTER, cpuid);

function where the local timers are emulated by a global always-on timer.
On CPU resume, the per-CPU tick device normal mode is restored by exiting
broadcast mode through

clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_EXIT, cpuid);

In order for this mechanism to function, architected timers should add to
their feature C3STOP, which means that they are not able to function when the
CPU is in off-mode.
Signed-off-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: NMarc Zyngier <marc.zyngier@arm.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>