dt-bindings: arm: imx: add imx8mm gw7901 support

dt-bindings: arm: imx: add imx8mm gw7901 support

The Gateworks GW7901 is an ARM based single board computer (SBC)
featuring:
 - i.MX8M Mini SoC
 - LPDDR4 DRAM
 - eMMC FLASH
 - SPI FRAM
 - Gateworks System Controller (GSC)
 - Atmel ATECC Crypto Authentication
 - USB 2.0
 - Microchip GbE Switch
 - Multiple multi-protocol RS232/RS485/RS422 Serial ports
 - onboard 802.11ac WiFi / BT
 - microSD socket
 - miniPCIe socket with PCIe, USB 2.0 and dual SIM sockets
 - Wide range DC power input
 - 802.3at PoE

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>