powerpc/85xx: use fdt_create_phandle() to create the Fman firmware phandles

Function fdt_create_phandle() conveniently creates new phandle properties
using both "linux,phandle" and "phandle", so it should be used by all code
that wants to create a phandle.

The Fman firmware code, which embeds an Fman firmware into the device tree,
was creating the phandle properties manually.  Instead, change it to use
fdt_create_phandle().

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