mfd: db8500: Update cpufreq device name

Since the cpufreq driver for ux500 has been renamed from
cpufreq-db8500 to cpufreq-dbx500, we need to change the
device name here as well.

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