arm64: dts: imx: Add i.MX8M Plus Gateworks gw7400 dts support

The Gateworks GW7400 is an ARM based single board computer (SBC)
featuring:
 - i.MX8M Plus SoC
 - LPDDR4 DRAM
 - eMMC FLASH
 - Gateworks System Controller (GSC)
 - QOS GbE
 - Microchip GbE Switch
 - Multiple multi-protocol RS232/RS485/RS422 Serial ports
 - USB 3.0 Front panel connector
 - onboard 802.11ac WiFi / BT
 - 3x miniPCIe socket with PCIe and USB 2.0
 - 1x M.2 B/A-E socket with PCIe, USB 3.0 and dual nano-SIM sockets
 - off-board connectors for: MIPI DSI, MIPI CSI, SPI, GPIO, I2C
 - Wide range DC power input
 - Passive PoE
 - 802.3at PoE

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Krzysztof Kozlowski <krzk+dt@kernel.org>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP Linux Team <linux-imx@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>