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/drivers/spi/zynq_spi.c b/drivers/spi/zynq_spi.c
index 2a02942..0b2b2f4 100644
--- a/drivers/spi/zynq_spi.c
+++ b/drivers/spi/zynq_spi.c
@@ -10,6 +10,7 @@
 #include <dm.h>
 #include <malloc.h>
 #include <spi.h>
+#include <time.h>
 #include <asm/io.h>
 
 DECLARE_GLOBAL_DATA_PTR;