post: fix compile issue for post tests on kirkwood

commit f31a911f (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>