dt-bindings: mmc: Remove redundant sdhci.txt

The properties in sdhci.txt are documented in sdhci-common.yaml, too.
Remove the txt binding.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20250320211922.1135669-1-robh@kernel.org


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>