Make converting the speed bin value into a speed grade generic and
determined by a platform specific callback. Also change the prototypes
involved to encode the speed bin directly in the return value.

This allows to extend the driver more easily to support more SoCs.
Signed-off-by: Brandon Cheo Fusi <fusibrandon13@gmail.com>
[Andre: merge output into return value]
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>