dt-bindings: mmc: update mmc-card.yaml reference

The recent change ("dt-bindings: mmc: Convert MMC Card binding to a schema")
renamed: Documentation/devicetree/bindings/mmc/mmc-card.txt
to: Documentation/devicetree/bindings/mmc/mmc-card.yaml.

Let's update its cross-reference accordingly.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/820bb7a1d7e0e51cbea72c9bee6bce806427d1f3.1631785820.git.mchehab+huawei@kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>