post: fix compile issue for post tests on kirkwood

commit f31a911fe (arm, post: add missing post_time_ms for arm)
enables get_ticks and get_tbclk for all arm based boards,
but kirkwood has currently no implementation for this. So
undefine this for kirkwood boards.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
cc: Heiko Schocher <hs@denx.de>
cc: Prafulla Wadaskar <prafulla@marvell.com>
cc: Valentin Longchamp <valentin.longchamp@keymile.com>

post/post.c

@@ -495,7 +495,7 @@ void post_reloc(void)
  */
 unsigned long post_time_ms(unsigned long base)
 {
-#if defined(CONFIG_PPC) || defined(CONFIG_ARM)
+#if defined(CONFIG_PPC) || defined(CONFIG_ARM) && !defined(CONFIG_KIRKWOOD)
 	return (unsigned long)lldiv(get_ticks(), get_tbclk() / CONFIG_SYS_HZ)
 		- base;
 #else