MX5/MX6: add missing get_ticks() and get_tbclk()

commit f31a911f (arm, post: add missing post_time_ms for arm)
enables get_ticks and get_tbclk for all arm based boards,
MX5/MX6 have not yet implemented.

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Dirk Behme <dirk.behme@de.bosch.com>
CC: Jason Liu <jason.hui@linaro.org>
CC: Marek Vasut <marek.vasut@gmail.com>