arm: dts: marvell: clearfog-gtr: add board-specific compatible strings

Most arm board have a board-specific compatible string that allows e.g.
userspace to match specific firmware variants or apply specific
policies.

Add board-specific properties to both variants of the Clearfog GTR:
- solidrun,clearfog-gtr-l8
- solidrun,clearfog-gtr-s4

Introduction of a common parent (e.g. "solidrun,clearfog-gtr") is
omitted for brevity.
Since announcement of the two products no additional variants were added
it is assumed that there will always be just two.

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