Only some old OpenFirmware implementations rely on default sizes. Any
FDT and modern implementation should have explicit properties. Make the
OF_ROOT_NODE_*_CELLS_DEFAULT defines private so we don't get any outside
users.

This also gets us one step closer to removing the asm/prom.h dependency on
Sparc.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Frank Rowand <frowand.list@gmail.com>
Cc: sparclinux@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>