hw/arm/raspi: Diagnose requests for too much RAM

hw/arm/raspi: Diagnose requests for too much RAM

The Raspberry Pi boards have a physical memory map which does
not allow for more than 1GB of RAM. Currently if the user tries
to ask for more then we fail in a confusing way:

$ qemu-system-aarch64 --machine raspi3 -m 8G
Unexpected error in visit_type_uintN() at qapi/qapi-visit-core.c:164:
qemu-system-aarch64: Parameter 'vcram-base' expects uint32_t
Aborted (core dumped)

Catch this earlier and diagnose it with a more friendly message:
$ qemu-system-aarch64 --machine raspi3 -m 8G
qemu-system-aarch64: Requested ram size is too large for this machine: maximum is 1GB

Fixes: https://bugs.launchpad.net/qemu/+bug/1794187
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>