docs: devicetree: add bindings for Seville DSA switch inside Felix driver

There are no non-standard bindings being used. However Felix is a PCI
device and Seville is a platform device. So give an example of device
tree for this switch and document its compatible string.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>