ARM: mmp: stop using mach/timex.h

mach/timex.h is the last remaining header that is unused for multiarch
builds but necessary for singlearch builds. To allow to get rid of it
for singlearch builds, too, drop its usage in mmp arch code by
substituting CLOCK_TICK_RATE by a local cpp symbol.

Acked-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>