dt-bindings: EXYNOS: Document compatibles from other vendors

Document compatibles used on other Exynos-based boards (non-Samsung):
FriendlyARM, Google, Hardkernel and Insignal.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Cc: Hakjoo Kim <ruppi.kim@hardkernel.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>