ARM: local timers: Add A15 architected timer support

Add support for the A15 generic timer and clocksource.
As the timer generates interrupts on a different PPI depending
on the execution mode (normal or secure), it is possible to
register two different PPIs.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

Conflicts:
        [Integrate to the recent patch which has changes to
         local timer registration mechanism.

         This fixes the crash seen during hotplug operations
         where after a secondary CPU is brought back online,
         the clock event device setup was happening as part
         of the online notification mechanism which was too
         late. With this change in the local timer mechanims,
         the clock event device is now setup as part of the
         secondary CPU boot initialization making it available
         early enough for use.

         Update the board file with the appropriate changes in
         the argument for timer registration.]
	arch/arm/Kconfig
	arch/arm/include/asm/arch_timer.h
	arch/arm/kernel/arch_timer.c

Change-Id: I0bc80097c145fb2aac2150db0c5dff3c5e215a58
Signed-off-by: Sathish Ambley <sambley@codeaurora.org>