spapr: Use QEMU limit for maximum CPUs number

sPAPR uses hard coded limit of maximum 255 supported CPUs which is
exactly the same as QEMU-wide limit which is MAX_CPUMASK_BITS and also
defined as 255.

This makes use of a global CPU number limit for the "pseries" machine.

In order to anticipate future increase of the MAX_CPUMASK_BITS
(or to help debugging large systems), this also bumps the FDT_MAX_SIZE
limit from 256K to 1M assuming that 1 CPU core needs roughly 512 bytes
in the device tree so the new limit can cover up to 2048 CPU cores.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>