mfd: db8500-prcmu: Support the higher DB8520 ARMSS

The DB8520 used in a lot of Samsung phones has a slightly higher
maximum ARMSS frequency than the DB8500. In order to not confuse
the OPP framework and cpufreq, make sure the PRCMU driver
returns the correct frequency.

Cc: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>