Add support for the i.MX7D SMEGW01 board.

This is a gateway board that supports the following peripherals:

- eMMC / SD card
- RTC
- USB modem
- Wifi via SDIO
- Dual Ethernet
- CAN
- Serial SRAM

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>