cpufreq: dbx500: Round to closest available freq

When reading the cpu speed, round it to the closest available
frequency from the table.

Signed-off-by: Mats Fagerstrom <mats.fagerstrom@stericsson.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>