Convert the marvell,xenon-sdhci binding to JSON schema. Currently the
in-tree dts files don't validate because they use sdhci@ instead of mmc@
as required by the generic mmc-controller schema.

The compatible "marvell,sdhci-xenon" was not documented in the old
binding but it accompanies the of "marvell,armada-3700-sdhci" in the
armada-37xx SoC dtsi so this combination is added to the new binding
document.
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220329220544.2132135-3-chris.packham@alliedtelesis.co.nzSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>