imx6: add support for GW5907/GW5910/GW5912/GW5913

imx6: add support for GW5907/GW5910/GW5912/GW5913

This patch adds support for GW5907/GW5910/GW5912/GW5913 IMX6 based boards
from the Gateworks Ventana Family[A]:
- backport upstream dt patches from 5.6 to 4.19 and 5.4
- add dtb's to ventana images
- add board-name and network config

A. https://www.gateworks.com/products/imx6-single-board-computer-gateworks-ventana-family

Flashing instructions for Ventana boards:

Using pre-flashed bootloader:
- Use appropriate ubi image depending on board NAND to flash via bootloader:
	openwrt-imx6-ventana-squashfs-nand.ubi - 2KiB page size
	openwrt-imx6-ventana-large-squashfs-nand.ubi - 4KiB page size
	http://trac.gateworks.com/wiki/linux/ubi

Using Gateworks JTAG dongle:
- Use Gateworks mkimage_jtag script to create a JTAG image comprised of
	pre-built bootloader and ubi image:
	http://trac.gateworks.com/wiki/jtag_instructions

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