dt-bindings: arm: fsl: Document Variscite i.MX6q devicetree

Document the Variscite i.MX6qdl board devicetree binding
already supported:

- variscite,dt6customboard

Signed-off-by: Oliver Graute <oliver.graute@kococonnector.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>