ARM: ux500: fix the smp_twd clock calculation

The clock for the smp_twd block is not equal to the CPU
frequency, actually it is divided by two, so fix this,
and set the initial frequency to half of 1GHz which is
the most common case.

Reported-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>