cpufreq: update DB8500 cpufreq driver

This updates the ux500 cpufreq driver to the new interface from the
updated DB8500 PRCMU

Cc: Arnd Bergmann <arnd@arndb.de>
Acked-by: Dave Jones <davej@redhat.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>