board: gateworks: venice: add imx8mm-gw7902 support

board: gateworks: venice: add imx8mm-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

Do the following to add support for it:
- add dts
- add PMIC config

Signed-off-by: Tim Harvey <tharvey@gateworks.com>