[ARM] rtc-sa1100: don't assume CLOCK_TICK_RATE to be a constant

As Nicolas and Russell pointed out, CLOCK_TICK_RATE is no more
a constant on PXA when multiple processors and platforms are
selected, change TIMER_FREQ in rtc-sa1100.c into a variable.

Since the code to decide the clock tick rate is re-used from
timer.c, introduce a common get_clock_tick_rate() for this.

Signed-off-by: Eric Miao <eric.miao@marvell.com>
Acked-by: Nicolas Pitre <nico@marvell.com>
diff --git a/arch/arm/mach-pxa/time.c b/arch/arm/mach-pxa/time.c
index f8a9a62..986d494 100644
--- a/arch/arm/mach-pxa/time.c
+++ b/arch/arm/mach-pxa/time.c
@@ -23,7 +23,6 @@
 #include <asm/mach/irq.h>
 #include <asm/mach/time.h>
 #include <mach/pxa-regs.h>
-#include <asm/mach-types.h>
 
 /*
  * This is PXA's sched_clock implementation. This has a resolution
@@ -150,18 +149,11 @@
 
 static void __init pxa_timer_init(void)
 {
-	unsigned long clock_tick_rate;
+	unsigned long clock_tick_rate = get_clock_tick_rate();
 
 	OIER = 0;
 	OSSR = OSSR_M0 | OSSR_M1 | OSSR_M2 | OSSR_M3;
 
-	if (cpu_is_pxa25x())
-		clock_tick_rate = 3686400;
-	else if (machine_is_mainstone())
-		clock_tick_rate = 3249600;
-	else
-		clock_tick_rate = 3250000;
-
 	set_oscr2ns_scale(clock_tick_rate);
 
 	ckevt_pxa_osmr0.mult =