hw/arm/raspi: Extract the RAM size from the board revision

hw/arm/raspi: Extract the RAM size from the board revision

The board revision encode the amount of RAM. Add a helper
to extract the RAM size, and use it.
Since the amount of RAM is fixed (it is impossible to physically
modify to have more or less RAM), do not allow sizes different
than the one anounced by the manufacturer.

Acked-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20200208165645.15657-5-f4bug@amsat.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>