cpufreq: add driver for Raspberry Pi

Raspberry Pi's firmware offers and interface though which update it's
performance requirements. It allows us to request for specific runtime
frequencies, which the firmware might or might not respect, depending on
the firmware configuration and thermals.

As the maximum and minimum frequencies are configurable in the firmware
there is no way to know in advance their values. So the Raspberry Pi
cpufreq driver queries them, builds an opp frequency table to then
launch cpufreq-dt.

Also, as the firmware interface might be configured as a module, making
the cpu clock unavailable during init, this implements a full fledged
driver, as opposed to most drivers registering cpufreq-dt, which only
make use of an init routine.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Acked-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Acked-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>