board: gateworks: venice: add imx8mn-gw7902 support

board: gateworks: venice: add 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

To add support for the i.MX8M Nano GW7902:
 - Add imx8mn-venice dts/defconfig/include
 - Add imx8mn-gw7902 dts
 - Add imx8mn-2gb lpddr4 dram configs
 - Add misc support for IMX8M Nano SoC
 - rename imx8mm-venice.c to venice.c as it is no longer imx8mm specific
 - update README with differences for IMX8MN vs IMX8MM

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