ppc: move New World uninorth and nvram device node creation to the root device

Whilst the NVRAM device node exists under the mac-io device node for Old World
Macs, both itself and the Uninorth device node exist under the root device node
on New World Macs.

Move creation of both device nodes to arch_of_init() for New World machines to
enable the active package and current instance to be set correctly during device
tree construction.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>