ARM: dts: imx7d-pico-pi: Move SoM related part to imx7d-pico.dtsi

imx7d-pico-pi board contains:

- One SoM board (imx7d pico)
- One base board (pi).

In order to make it easier for adding support for other board variants,
move the commom SoM part to the imx7d-pico.dtsi file.

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>