dt-bindings: mmc: convert mmc-spi-slot to yaml

Convert the mmc-spi-slot text based binding doc to yaml,
with the side effect of cleaning up some of the riscv
dtbs_check warnings.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220606201343.514391-2-mail@conchuod.ie


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