ARM: dts: imx6qdl: Add Variscite VAR-SOM-MX6 SoM support

ARM: dts: imx6qdl: Add Variscite VAR-SOM-MX6 SoM support

This patch adds support for the Variscite VAR_SOM-MX6 SoM with :
- i.MX6 Quad or Dual Lite SoC
- 256 – 4096 MB DDR3
- 4-64 GB eMMC
- 128 – 1024 MB SLC NAND
- Camera Interface
- HDMI+CEC interface
- LVDS / DSI / Parallel RGB interfaces
- Ethernet RGMII interface
- On-SoM Wi-Fi/Bluetooth with WiLink wl183x 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 : https://www.variscite.com/product/system-on-module-som/cortex-a9/var-som-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.

This file is based on the one provided by Variscite on their own
kernel, but adapted for mainline.

Cc: Pierluigi Passaro <pierluigi.p@variscite.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>