dt-bindings: arm: Add Gateworks i.MX8M Plus gw71xx-2x board

Add DT compatible string for a Gateworks GW71xx-2x board based on
the i.MX8M Plus SoC from NXP.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>