ARM: dts: imx7d: recreate imx7d.dtsi with i.MX 7Dual specifics

The i.MX 7Solo implements a subset of features available on
i.MX 7Dual. Recreate imx7s.dtsi as the base device tree for
i.MX 7Dual boards. The i.MX 7Dual's additional features over
i.MX 7Solo are:
- Second Cortex-A7 core
- Second Gigabit Ethernet controller
- EPD (Electronc Paper Display, not yet part of the device tree)
- PCIe (not yet part of the device tree)
- Additional USB2.0 OTG controller

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