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.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
diff --git a/arch/arm/include/asm/arch_timer.h b/arch/arm/include/asm/arch_timer.h
index 827305d..dc008c6 100644
--- a/arch/arm/include/asm/arch_timer.h
+++ b/arch/arm/include/asm/arch_timer.h
@@ -9,11 +9,17 @@
 
 #ifdef CONFIG_ARM_ARCH_TIMER
 int arch_timer_register(struct arch_timer *);
+int arch_timer_sched_clock_init(void);
 #else
 static inline int arch_timer_register(struct arch_timer *at)
 {
 	return -ENXIO;
 }
+
+static inline int arch_timer_sched_clock_init(void)
+{
+	return -ENXIO;
+}
 #endif
 
 #endif