dt-bindings: MIPS: Document xilfpga bindings and boot style

Xilfpga boots only with device-tree. Document the required properties
and the unique boot style

Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Cc: robh+dt@kernel.org
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/11361/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>