dt-bindings: fsi: Document the FSI Hub Controller

Document the FSI Hub Controller CFAM engine.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240522192524.3286237-11-eajames@linux.ibm.com


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>