ARM: dts: imx: Add GW5912 board support

The Gateworks GW5912 is an IMX6 SoC based single board computer with:
 - IMX6Q or IMX6DL
 - 32bit DDR3 DRAM
 - GbE RJ45 front-panel
 - 4x miniPCIe socket with PCI Gen2, USB2
 - 1x miniPCIe socket with PCI Gen2, USB2, mSATA
 - 1x miniPCIe socket with PCI Gen2, USB2, mezzanine
 - 10V to 60V DC input barrel jack
 - 3axis accelerometer (lis2de12)
 - GPS (ublox ZOE-M8Q)
 - bi-color front-panel LED
 - 256MB NAND boot device
 - nanoSIM/microSD socket (with UHS-I support)
 - user pushbutton
 - Gateworks System Controller (hwmon, pushbutton controller, EEPROM)
 - CAN Bus transceiver (mcp2562)
 - RS232 transceiver (1x UART with flow-control or 2x UART (build option)
 - off-board SPI connector (1x chip-select)

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