Replace the FIXME by appropriate description.

Fixes: 39bd2b6a ("dt-bindings: Improve phandle-array schemas")
Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220505113639.1344281-1-arnaud.pouliquen@foss.st.comSigned-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>