arm64: dts: marvell: Update sdhci node names to match schema

Update the node names of the sdhci@ interfaces to be mmc@ to match the
node name enforced by the mmc-controller.yaml schema.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>