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

The rpc timekeeping code uses the symbol LATCH which depends on
CLOCK_TICK_RATE which is defined in rpc's <mach/timex.h>. As this header
will go away in a later patch introduce a local variable holding the
same value as LATCH and use that instead.

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