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/mach-rmobile/timer.c b/arch/arm/mach-rmobile/timer.c
index bf74955..9fcab44 100644
--- a/arch/arm/mach-rmobile/timer.c
+++ b/arch/arm/mach-rmobile/timer.c
@@ -6,6 +6,7 @@
 
 #include <common.h>
 #include <div64.h>
+#include <time.h>
 #include <asm/io.h>
 #include <asm/arch-armv7/globaltimer.h>
 #include <asm/arch/rmobile.h>