Platform/RPi/ConfigDxe: Improve CPU Frequency configuration

Improve the CPU frequency settings of the platforms by:
- Adding a "Default" option that sets the frequency to the
  official default for each model/submodel.
- Adding a "Low" option, that sets the frequency to a fixed
  PCD, custom to each platform.
- Using fixed PCDs to set the maximum and default values for
  the custom frequency range, according to the overclocking
  capabilities of the platform.
- Ensuring that the firmware defaults to using the platform's
  default frequency, instead of a low arbitrary value.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>