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: NRob Herring <rob.herring@calxeda.com>

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

arch/arm/kernel/sched_clock.c

@@ -165,12 +165,19 @@ void __init setup_sched_clock(u32 (*read)(void), int bits, unsigned long rate)
 	pr_debug("Registered %pF as sched_clock source\n", read);
 }
 
-unsigned long long notrace sched_clock(void)
+static unsigned long long notrace sched_clock_32(void)
 {
 	u32 cyc = read_sched_clock();
 	return cyc_to_sched_clock(cyc, sched_clock_mask);
 }
 
+unsigned long long __read_mostly (*sched_clock_func)(void) = sched_clock_32;
+
+unsigned long long notrace sched_clock(void)
+{
+	return sched_clock_func();
+}
+
 void __init sched_clock_postinit(void)
 {
 	/*