mfd: db8500: Connect ARMSS clk to ARM OPP

ARMSS clk directly maps it's frequency towards the cpufreq table.
To be able to update the ARMSS clk rate, a new set_rate function for
the ARMSS clk is added, which also will trigger a corresponding ARM
OPP request. Additionally an ARMSS clk round_rate function is added
to fetch valid cpufreq frequencies.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>