ARM: dts: Add ZII support for ZII i.MX7 RMU2 board

Add support for ZII's i.MX7 based Remote Modem Unit 2 (RMU2) board.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Chris Healy <cphealy@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Bob Langer <Bob.Langer@zii.aero>
Cc: Liang Pan <Liang.Pan@zii.aero>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Cc: devicetree@vger.kernel.org
Signed-off-by: Shawn Guo <shawnguo@kernel.org>