arm64: dts: marvell: Use a SoC-specific compatible for xHCI on Armada37xx

Even if the Armada 37xx does not any specific setup, the device tree
binding documentation requires to use a SoC-specific version
corresponding to the platform first followed by the generic version.

This patch introduce this new compatible string and updates the
documentation accordingly.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>