dtc: change default phandles to ePAPR style instead of both

Currently, both legacy (linux,phandle) and ePAPR (phandle) properties
are inserted into dtbs by default. The newer ePAPR style has been
supported in dtc and Linux kernel for 7 years. That should be a long
enough transition period. We can save a little space by not putting both
into the dtb.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>