dt-bindings: mmc: controller: clarify the address-cells description

The term "slot ID" has nothing to do with the SDIO function number
which is specified in the reg property of the subnodes, rephrase
the description to be more accurate.

Fixes: f9b79898 ("dt-bindings: mmc: Add YAML schemas for the generic MMC options")
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Message-ID: <20241128-topic-amlogic-arm32-upstream-bindings-fixes-convert-meson-mx-sdio-v4-1-11d9f9200a59@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>