ARM: dts: imx6ul: Add Variscite VAR-SOM-MX6UL SoM support

ARM: dts: imx6ul: Add Variscite VAR-SOM-MX6UL SoM support

Add support for the Variscite VAR_SOM-MX6UL SoM with:

- NXP i.MX6 UltraLite SoC
- 128–1024 MB DDR3L
- 8–128 GB eMMC
- 128–512 MB SLC NAND
- Camera Interface
- LVDS / Parallel RGB interfaces (not configured)
- Touch controller (not configured)
- Ethernet RMII interface (not configured)
- On-SoM Wi-Fi/Bluetooth with CYW43353 on SDIO Module (not configured)
- SD/MMC/SDIO interface
- USB Host + USB OTG interface
- I2C interfaces
- SPI interfaces
- PCI-Express 2.0 interface
- on-SoM Audio Codec (not configured)
- S/PDIF interface (not configured)

Product website: https://www.variscite.com/product/system-on-module-som/cortex-a7/var-som-6ul-nxp-imx6ul-6ull-6ulz

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

I tested this on a VAR-SOM-6UL_G2_700C_512R_8N_IT_REV1.3A, which is why
some of the features above are mentioned as "not configured" (I couldn't
test them).

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

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>