arm64: dts: imx: Add i.mx8mm Gateworks gw7904 dts support

The GW7904 is based on the i.MX 8M Mini SoC featuring:
 - LPDDR4 DRAM
 - eMMC FLASH
 - microSD connector with UHS support
 - LIS2DE12 3-axis accelerometer
 - Gateworks System Controller
 - IMX8M FEC
 - 2x RS232 off-board connectors
 - PMIC
 - 10x bi-color LED's
 - 1x miniPCIe socket with PCIe and USB2.0
 - 802.3at Class 4 PoE
 - 10-30VDC input via barrel-jack

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