dt-bindings: fsi: Add P9 OCC device documentation

Device-tree bindings for FSI-attached POWER9 On-Chip Controller (OCC)

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This is the binding for the P9 On-Chip Controller accessed over FSI from a

service processor. See fsi.txt for details on bindings for FSI slave and CFAM

nodes. The OCC is not an FSI slave device itself, rather it is accessed

through the SBE fifo.

Required properties:

 - compatible = "ibm,p9-occ"

Examples:

    occ {

        compatible = "ibm,p9-occ";

    };