imx6: add GW553x support

imx6: add GW553x support

The Gateworks Ventana GW553x is a single-board computer based on the NXP
IMX6 SoC with the following features:
 * IMX6 DualLite Soc (supports IMX6S,IMX6DL,IMX6Q)
 * small form factor (35x70mm)
 * 512MB DDR3 DRAM (2x32bit) (options up to 2GB)
 * 256MB NAND flash (4GB option)
 * Gateworks System Controller:
  - hardware watchdog
  - hardware monitor
  - pushbutton controller
  - EEPROM storage
  - power control
 * JTAG programmable
 * 1x miniPCI socket (with PCIe, USB, SIM)
 * 1x microSD socket
 * 1x SIM socket
 * Inertial Module (LSM9DS1 9DOF: 3x acc, 3x rate, 3x mag)
 * analog CVBS video in
 * GPS (optional uBlox EVA-M8M)
 * Application headers:
  - 2x TTL UART (TX/RX)
  - 4x TTL GPIO (3x configurable as PWM)
  - 3x CVBS Analog video input (1x decoder with 3x selectable inputs)
 * Front panel connectors:
  * micro-HDMI audio/video out
  - 1x user programmable LED
  - 1x configurable user pushbutton
  - 1x USB OTG

See http://www.gateworks.com for more info

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