Add compatible string for imx7ulp evk board.

Cc: devicetree@vger.kernel.org
Cc: Shawn Guo <shawnguo@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>