ARM: ixp4xx: avoid circular header dependency

With the new linux/reboot.h header file dependency added, we can no
longer build ixp4xx. The easiest way to avoid that is to remove the
inclusion of mach/hardware.h from mach/timex.h, which does not need
that header anyway.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: Krzysztof Halasa <khc@pm.waw.pl>
Cc: Jason Cooper <jason@lakedaemon.net>