hw/arm/raspi: Set default RAM size to size encoded in board revision

hw/arm/raspi: Set default RAM size to size encoded in board revision

We added a helper to extract the RAM size from the board
revision, and made board_rev a field of RaspiMachineClass.
The class_init() can now use the helper to extract from the
board revision the board-specific amount of RAM.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20200208165645.15657-11-f4bug@amsat.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>