cpufreq: make DB8500 cpufreq driver compile

Concluding interface update and movement of the driver by making
the DB8500 cpufreq driver compile in the cpufreq subsystem.

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