hw/openrisc/openrisc_sim: Increase max_cpus to 4

Now that we no longer have a limit of 2 CPUs due to fixing the
IRQ routing issues we can increase the max.  Here we increase
the limit to 4, we could go higher, but currently OMPIC has a
limit of 4, so we align with that.

Signed-off-by: Stafford Horne <shorne@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>