ARM: dts: imx6ul-pico: Add the imx6ul-pico-pi variant

The imx6ul-pico-pi contains a imx6ul-pico SoM and a pi baseboard:
https://www.technexion.com/products/pico-baseboards/detail/PICO-PI



Add support for it.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>