ARM: tegra: Handle timers during LP2 idle ticks

ARM: tegra: Handle timers during LP2 idle ticks

Timer ticks aren't properly serviced while a CPU is in LP2 idle.
Although the Tegra LP2 idle code calls hrtimer_peek_ahead_timers,
because no IRQ regs have been saved, update_process_times is not
called, and thus the timer list is not serviced (and neither is
SMP rebalancing, etc.)  This can cause significant delays
scheduling timer-based activity, especially on CPU 1 (which is
not servicing most other IRQs).

Colin Cross suggested a patch based on upstream review feedback
that uses clock notifiers to switch to the "broadcast" clock event
source ("timer0" Tegra timer 3) during LP2, which has a real
interrupt handler defined that calls the clock event handler in
IRQ context, allowing timers to be checked.

Change-Id: Ifa3f4ec662f07dc9636e433f278358f75b65d10c
Signed-off-by: Todd Poynor <toddpoynor@google.com>