raspi: Add "raspi3" machine type

raspi: Add "raspi3" machine type

This patch adds a "raspi3" machine type, which can now be selected as
the machine to run on by users via the "-M" command line option to QEMU.

The machine type does *not* ignore memory transaction failures so we
likely need to add some dummy devices later when people run something
more complicated than what I'm using for testing.

Signed-off-by: Pekka Enberg <penberg@iki.fi>
[PMM: added #ifdef TARGET_AARCH64 so we don't provide the 64-bit
 board in the 32-bit only arm-softmmu build.]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>