dt-bindings: arm: fsl: add IMX8MN DDR3L eval board

This eval board features an IMX8MN UltraLite and has DDR3L RAM. The
product part number is 8MNANOD3L-EVK.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Heiko Thiery <heiko.thiery@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Acked-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>