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/tegra20_sflash.c b/drivers/spi/tegra20_sflash.c
index 567e33f..0e68d33 100644
--- a/drivers/spi/tegra20_sflash.c
+++ b/drivers/spi/tegra20_sflash.c
@@ -8,6 +8,7 @@
 #include <common.h>
 #include <dm.h>
 #include <errno.h>
+#include <time.h>
 #include <asm/io.h>
 #include <asm/gpio.h>
 #include <asm/arch/clock.h>