common: Move some time functions out of common.h

These functions belong in time.h so move them over and add comments.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/arm/mach-imx/timer.c b/arch/arm/mach-imx/timer.c
index ed5eb1c..5fe5c51 100644
--- a/arch/arm/mach-imx/timer.c
+++ b/arch/arm/mach-imx/timer.c
@@ -7,6 +7,7 @@
  */
 
 #include <common.h>
+#include <time.h>
 #include <asm/io.h>
 #include <div64.h>
 #include <asm/arch/imx-regs.h>