dt-bindings: fsi: Document the AST2700 FSI controller

Add the appropriate compatible string, and document the new reg
properties of the AST2700 FSI controller, which can directly access
the FSI controller registers as well as the FSI link address space.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240522192524.3286237-10-eajames@linux.ibm.com


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