imx6: kernel: add GW5520 support

The GW5520 is a small form-factor single-board computer with the following
features:
 * 70x100mm form-factor
 * IMX6DL 800MHz SoC (IMX6Q optional)
 * 512MB 32bit DDR3 SDRAM (up to 2GB optional)
 * 256MB NAND FLASH (up to 2GB optional)
 * Gateworks System Controller
 * 2x front-panel Intel i210 GbE adapters with passive PoE support
 * 2x MiniPCIe sockets with USB support
 * 2x front-panel USB
 * 1x rear-panel full-size HDMI connector
 * 1x front-panel bi-color user LED
 * 1x front-panel user pushbutton
 * 1x rear-panel barrel jack for power
 * 1x Application connector with:
  * 2x TTL level UARTs
  * 10x TTL level Digital IO

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42148 3c298f89-4303-0410-b956-a3cf2f4a3e73