cpufreq: db8500: Register as a platform driver

To fetch the mfd child device we register the cpufreq driver
as a platform driver.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Jonas Aaberg <jonas.aberg@stericsson.com>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>