pico-imx7d: Add initial support

Add the initial support for pico-imx7d board based on Wig Cheng's
source code.

Add support for eMMC, USB gadget, I2C, PMIC and Ethernet.

For more information about this board, please visit:
http://www.technexion.org/products/pico/pico-som/pico-imx7-emmc



Signed-off-by: Vanessa Maegima <vanessa.maegima@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>