ARM: architected timers: Add A15 specific sched_clock implementation

Provide an A15 sched_clock implementation using the virtual counter,
which is thought to be more useful than the physical one in a
virtualised environment, as it can offset the time spent in another
VM or the hypervisor.

Change-Id: Ica870d279dba38304581763654c683cd09f87153
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
[sambley@codeaurora.org Fix conflicts due to patched code not
against latest version of arch_timer.c]
Signed-off-by: Sathish Ambley <sambley@codeaurora.org>