ARM: dts: imx7d-pico-pi: Separate into cpu and baseboard dts

Separate the old imx7d-pico into cpu (imx7d-pico.dtsi) and baseboard
(imx7d-pico-pi.dts) dts so the same cpu dtsi can be used in different
baseboards variants.

Signed-off-by: Vanessa Maegima <vanessa.maegima@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>