rtc: at91sam9: include <mach/hardware.h> explicitly

The driver needs the symbol AT91_SLOW_CLOCK which is defined in
arch/arm/mach-at91/include/mach/hardware.h. This file is included
implicitly via linux/module.h -> linux/kmod.h -> linux/gfp.h ->
linux/mmzone.h -> linux/memory_hotplug.h -> linux/notifier.h ->
linux/srcu.h -> linux/workqueue.h -> linux/timer.h -> linux/ktime.h ->
linux/jiffies.h -> linux/timex.h -> mach/timex.h -> mach/hardware.h .

So better include it explicitly not only because the last link will go
away soon.

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