Reflect in the binding that the cd interrupt is optional,
and add a description of the interrupts.

Fixes: 09b31a61 ("dt-bindings: mmc: meson-gx: support specifying cd interrupt")
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/c1437eb6-0c01-809f-422b-cb1109489355@gmail.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>