dt-bindings: Document mti,mips-cpc binding

Document a binding for the MIPS Cluster Power Controller (CPC) that
allows the device tree to specify where the CPC registers are located.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Signed-off-by: Aleksandar Markovic <aleksandar.markovic@mips.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/18512/
Signed-off-by: James Hogan <jhogan@kernel.org>