ARM: dts: imx: Add GW5907 board support

The Gateworks GW5907 is an IMX6 SoC based single board computer with:
 - IMX6Q or IMX6DL
 - 32bit DDR3 DRAM
 - FEC GbE Phy
 - bi-color front-panel LED
 - 256MB NAND boot device
 - Gateworks System Controller (hwmon, pushbutton controller, EEPROM)
 - Digital IO expander (pca9555)
 - Joystick 12bit adc (ads1015)

Signed-off-by: Robert Jones <rjones@gateworks.com>
Reviewed-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>