fdt: update fdt_alloc_phandle to use fdt_get_phandle

fdt: update fdt_alloc_phandle to use fdt_get_phandle

The device tree compiler, dtc, can use "phandle" and/or "linux,phandle"
properties to specify the phandle for any node.  By default, it uses
both, but "linux,phandle" is deprecated.  One day, we'd like to stop using
"linux,phandle", but U-boot needs to support both properties equally
first.

fdt_alloc_phandle() generates a unique phandle, but it was only checking
the "linux,phandle" properties.  Instead, we use fdt_get_phandle(),
which checks both properties automatically.  This ensures that we
support dtbs that only use "phandle".

The side-effect is that fdt_alloc_phandle() now takes twice as long, since
it has to check for two properties instead of one in each node that it
searches.

Signed-off-by: Timur Tabi <timur@freescale.com>