dt-bindings: mmc: document mmc-slot

Document the mmc-slot, which is a subnode of a multi-slot
MMC controller, each slot is represented as a full MMC controller,
the top node handling all the shared resources and slot mux.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Message-ID: <20241128-topic-amlogic-arm32-upstream-bindings-fixes-convert-meson-mx-sdio-v4-4-11d9f9200a59@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>