hw/arm/raspi: Use more specific machine names

hw/arm/raspi: Use more specific machine names

Now that we can instantiate different machines based on their
board_rev register value, we can have various raspi2 and raspi3.

In commit fc78a990ec103 we corrected the machine description.
Correct the machine names too. For backward compatibility, add
an alias to the previous generic name.

Reviewed-by: Luc Michel <luc.michel@greensocs.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20200924111808.77168-6-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>