dt-bindings: net: move FMan binding

Besides the PPC SoCs, the QorIQ DPAA FMan is also present on ARM SoCs,
moving the device tree binding document into the bindings/net folder.
Signed-off-by: Madalin Bucur <madalin.bucur@nxp.com>
Signed-off-by: Rob Herring <robh@kernel.org>