arm: dts: marvell: clearfog: add pro variant compatible in legacy dts

Armada 388 Clearfog ("armada-388-clearfog.dts)" is a legacy filename for
the Armada 388 Clearfog Pro ("armada-388-clearfog-pro.dts").
The "Pro" suffix was only used when the smaller version, the "Base" got
released.

The two names refer to exactly the same hardware, therefore they should
share the same compatible strings.

Copy "solidrun,clearfog-pro-a1" compatible from the -pro dts and add it
to this legacy file.

Signed-off-by: Josua Mayer <josua@solid-run.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>