clocksource: nomadik-mtu: fix up clocksource/timer

The Nomadik clocksource driver has had a bad define making it
impossible to use it for sched_clock() for a while. Fix this
and also enable it for the Nomadik.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>