versatilepb: do not run if user asks for more than 256MB RAM

The versatilepb physical address space layout only has
a 256MB region for RAM before the devices. Without a guard
on the amount of RAM requested by the user we would happily
create a RAM area that overlapped with the devices, resulting
in very confusing behaviour (typically a guest crash).

Report the problem to the user if they try to request more
RAM than the board can handle (as we do already for some
other board models).

Signed-off-by: Jean-Christophe Dubois <jcd@tribudubois.net>
Message-id: 20161025093711.17407-1-jcd@tribudubois.net
[PMM: tidied up commit message, comments. Use error_report()
 rather than fprintf(stderr, ...).]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
