cpufreq: Give driver used for dbx500 family a more generic name

This driver doesn't only handle cpufreq functionality for the
db8500 anymore. There are new variants which rely on it too.
Let's make the name a bit more generic.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>