[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>

arch/arm/mach-pxa/include/mach/hardware.h

@@ -291,6 +291,8 @@
  */
 extern unsigned int get_memclk_frequency_10khz(void);
 
+/* return the clock tick rate of the OS timer */
+extern unsigned long get_clock_tick_rate(void);
 #endif
 
 #if defined(CONFIG_MACH_ARMCORE) && defined(CONFIG_PCI)