dt-bindings: mmc: convert sdhci-dove to JSON schema

Convert the sdhci-dove binding to JSON schema. The optional clocks
property was not in the original binding document but has been in the
dove.dtsi since commit 5b03df9a

 ("ARM: dove: switch to DT clock
providers").

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20220419024611.1327525-5-chris.packham@alliedtelesis.co.nz
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>