of/flattree: Make the kernel accept ePAPR style phandle information

Currently when processing flattened device trees, the kernel expects
the phandle in a property called "linux,phandle".  The ePAPR spec -
not being Linux specific - instead requires phandles to be encoded in
a property named simply "phandle".  This patch makes the kernel accept
either form when unflattening the device tree.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>