mach-ux500: register a clock for the SMP TWD

The SMP TWD on the ux500 will change frequency at the same time as
the CPU. Loop back the frequency presented from the CPU into a
clock that is looked up by the SMP TWD driver with the new cpufreq
notifier hook.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>