ARM: ux500: Rename dbx500 cpufreq code to be more generic

The cpufreq driver doesn't only handle the db8500 anymore. There
are new variants which rely on it too, so we've renamed the
driver to be more generic.

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