ARM: dts: imx7d: use imx7s.dtsi as base device tree

The i.MX 7 series currently consists of two SoCs: i.MX 7Solo and
7Dual. The former has a subset of features of the latter, hence
use imx7s.dtsi as the new base device tree. To keep diffstat nice,
just move imx7d.dtsi to imx7s.dtsi temporarily and recreate
imx7d.dtsi in a second commit.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>