ARM: dts: imx6sx: Add UDOO Neo support

Add initial device trees for UDOO Neo Basic, Extended and Full boards:
* Serial console is enabled, other serial ports are prepared.
* I2C based PMIC is enabled.
* Ethernet is enabled for Basic and Full.
* SDHC is enabled, with the SDIO_PWR GPIO modeled as a regulator.
* Both user LEDs are enabled, with the orange one reserved for the M4
  and with the SD card as default trigger for the red LED.

The decision on a board compatible string is deferred to later.

Cc: Ettore Chimenti <ettore.chimenti@udoo.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>