ARM: make sched_clock just call a function pointer

This converts sched_clock to simply a call to a function pointer in order
to allow overriding it. This will allow for use with 64-bit counters where
overflow handling is not needed.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
diff --git a/arch/arm/include/asm/sched_clock.h b/arch/arm/include/asm/sched_clock.h
index e3f7572..3d520dd 100644
--- a/arch/arm/include/asm/sched_clock.h
+++ b/arch/arm/include/asm/sched_clock.h
@@ -11,4 +11,6 @@
 extern void sched_clock_postinit(void);
 extern void setup_sched_clock(u32 (*read)(void), int bits, unsigned long rate);
 
+extern unsigned long long (*sched_clock_func)(void);
+
 #endif