arm64: enable generic clockevent broadcast

On platforms with power management capabilities, timers that are shut
down when a CPU enters deep C-states must be emulated using an always-on
timer and a timer IPI to relay the timer IRQ to target CPUs on an SMP
system.

This patch enables the generic clockevents broadcast infrastructure for
arm64, by providing the required Kconfig entries and adding the timer
IPI infrastructure.

Change-Id: I7bbec8bb8401188967c04f82124b5437e22e7c56
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Git-commit: 1f85008e74768a88e1ddb96cc1fe45bb2378166c
Git-Repo: git://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git


Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>