Revert "[POWERPC] Don't complain if size-cells == 0 in prom_parse()"

This reverts commit fd6e9d39.

Having #size-cells == 0 in a node indicates that things under the
node aren't directly accessible, and therefore we shouldn't try to
translate addresses for devices under the node into CPU physical
addresses.

Some drivers, such as the nvram driver for powermacs, rely on
of_address_to_resource failing if they are called for a node
representing a device whose resources aren't directly accessible
by the CPU.  These drivers were broken by commit fd6e9d39,
resulting in the "Lombard" powerbook hanging early in the boot
process.

Signed-off-by: Paul Mackerras <paulus@samba.org>