ARM: ux500: Don't attempt to enable the Nomadik System Timer twice

When booting with DT enabled we already call clocksource_of_init(),
which in turn calls the OF version of nmdk_timer_init().

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>