imx: add Gateworks Venice support

imx: add Gateworks Venice support

Add support for Gateworks Venice imx8m family of boards:
 - required kernel modules for on-board devices
 - image generation
 - initial network config
 - sysupgrade support

The resulting compressed disk image
(bin/targets/imx/cortexa53/openwrt-imx-cortexa53-gateworks_venice-squashfs-img.gz)
can be installed on a Gateworks venice board via U-Boot:

u-boot=> tftpboot $loadaddr openwrt-imx-cortexa53-gateworks_venice-squashfs-img.gz && \
         gzwrite mmc $dev $loadaddr $filesize

WARNING: this will overwrite any boot firmware on the eMMC user hardware
partition which if being used will brick your board requiring JTAG to
re-program boot firmware and recover

The compressed disk image contains the partition table and filesystems only
and that it is expected that boot firmware is installed properly on the
eMMC boot0 hardware partition. The easiest way to ensure this is to
use the Gateworks JTAG adapter/process to install the latest boot firmware
as follows from a Linux host:
  wget http://dev.gateworks.com/jtag/jtag_usbv4
  chmod +x jtag_usbv4
  wget http://dev.gateworks.com/venice/images/firmware-venice-imx8mm.bin
  sudo ./jtag_usbv4 -p firmware-venice-imx8mm.bin

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