dt-bindings: arm: imx: add imx8mm/imx8mn GW7902 support

dt-bindings: arm: imx: add imx8mm/imx8mn GW7902 support

The GW7902 is based on the i.MX 8M Mini / Nano SoC featuring:
 - LPDDR4 DRAM
 - eMMC FLASH
 - Gateworks System Controller
 - LTE CAT M1 modem
 - USB 2.0 HUB
 - M.2 Socket with USB2.0, PCIe, and dual-SIM
 - IMX8M FEC
 - PCIe based GbE
 - RS232/RS485/RS422 serial transceiver
 - GPS
 - CAN bus
 - WiFi / Bluetooth
 - MIPI header (DSI/CSI/GPIO/PWM/I2S)
 - PMIC

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