board: gateworks: venice: add imx8mm-gw7903 support

The GW7903 is based on the i.MX 8M Mini SoC featuring:
 - LPDDR4 DRAM
 - eMMC FLASH
 - microSD socket with voltage select support
 - Gateworks System Controller
 - M.2 A-E Socket with USB2.0 and PCIe
 - MiniPCIe Socket with PCIe, USB2.0, and SIM
 - IMX8M FEC
 - RS232/RS485/RS422 serial transceiver
 - LIS2DE12 3-axis accelerometer
 - front panel LED's
 - off-board isolated digital I/O
 - Wide range DC power input
 - 802.3at PoE
 - PMIC

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>