ARM: dts: imx6qdl: Add Variscite DART-MX6 SoM support

This patch adds support for the Variscite DART-MX6 SoM with :
- i.MX6 Quad or Dual Lite SoC
- 1Gb/2Gb LPDDR2
- 4-64 GB eMMC
- Camera Interface
- HDMI+CEC interface
- LVDS / DSI / Parallel RGB interfaces
- Ethernet RGMII interface
- On-SoM Wi-Fi/Bluetooth with WiLink wl1835 SDIO Module
- SD/MMC/SDIO interface
- USB Host + USB OTG interface
- I2C interfaces
- SPI interfaces
- PCI-Express 2.0 interface
- on-SoM Audio Codec with HP/Line-In interfaces + DMIC interface
- Digital Audio interface
- S/PDIF interface

Product website : http://www.variscite.com/products/system-on-module-som/cortex-a9/dart-mx6-cpu-freescale-imx6

Support is handled with a SoM-centric dtsi exporting the default interfaces
along the default pinmuxing to be enabled by the board dts file.

Only board-independent devices like WiFi, eMMC or PMIC are enabled in the dtsi.
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>