cpufreq: db8500: Use armss clk to update frequency

Using the armss clk to update the frequency makes the driver no more
directly dependant on the prmcu API.

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