ARM: at91: don't use <mach/timex.h>

The platform specific <mach/timex.h> will be removed in a later patch.
So move its only still used symbol to a different header specific for
the only machine still using it. Also add a few explicit includes of
<mach/hardware.h> that are implicitly available through <mach/timex.h>.

Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>