PXA: Fix missing get_tbclk() breaking vpac boards

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>

arch/arm/cpu/pxa/timer.c

@@ -94,3 +94,8 @@ void __udelay(unsigned long usec)
 	while (get_ticks() < tmp)	/* loop till event */
 		 /*NOP*/;
 }
+
+ulong get_tbclk(void)
+{
+	return TIMER_FREQ_HZ;
+}