dt-bindings: arm: imx: add imx8mm gw7903 support

The GW7903 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
 - software selectable RS232/RS485/RS422 serial transceiver
 - PMIC
 - 2x off-board bi-directional opto-isolated digital I/O
 - 1x M.2 A-E Key Socket and 1x MiniPCIe socket with USB2.0 and PCIe
 (resistor loading to route PCIe/USB2 between M.2 and MiniPCIe socket)

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