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/arm926ejs/armada100/timer.c b/arch/arm/cpu/arm926ejs/armada100/timer.c
index d2ecbd0..6c6948a 100644
--- a/arch/arm/cpu/arm926ejs/armada100/timer.c
+++ b/arch/arm/cpu/arm926ejs/armada100/timer.c
@@ -7,6 +7,7 @@
  */
 
 #include <common.h>
+#include <time.h>
 #include <asm/arch/cpu.h>
 #include <asm/arch/armada100.h>