fdt: introduce fdt_create_phandle()

The ePAPR specification says that phandle properties should be called
"phandle", and not "linux,phandle".  To facilitate the migration from
"linux,phandle" to "phandle", introduce function fdt_create_phandle(),
which creates a phandle in a given node.  For now, we create both the
"phandle" and "linux,phandle" properties.  A later version of this
function will remove support for "linux,phandle".

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