ARM: ixp4xx: stop using <mach/timex.h>

The only user of symbols defined in ixp4xx's <mach/timex.h> is common.c.
Fix that one up by moving the used #define into common.c directly and
add a local substitute for the global define LATCH which uses
CLOCK_TICK_RATE. This makes ixp4xx not to be a bar to dropping support
for <mach/timex.h>.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>