common: Move get_ticks() function out of common.h

This function belongs in time.h so move it over and add a comment.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/arm/cpu/sa1100/timer.c b/arch/arm/cpu/sa1100/timer.c
index 0fac5c1..c6b1b2c 100644
--- a/arch/arm/cpu/sa1100/timer.c
+++ b/arch/arm/cpu/sa1100/timer.c
@@ -11,6 +11,7 @@
 
 #include <common.h>
 #include <SA-1100.h>
+#include <time.h>
 
 static ulong get_timer_masked (void)
 {