ARM: dts: iwg20d-q7: Rework DT architecture

Since the same carrier board may host RZ/G1M and RZ/G1N based
Systems on Module, the DT architecture for iwg20d-q7 needs
better decoupling. This patch provides:
* iwg20d-q7-common.dtsi - its purpose is to define the carrier
  board definitions, and its content is basically the same
  as the previous version of r8a7743-iwg20d-q7.dts, only it
  has no reference to the SoM .dtsi, and that's why the
  filename doesn't mention the SoC name any more.
* r8a7743-iwg20d-q7.dts - its new purpose is to put together
  the SoM .dtsi (r8a7743-iwg20m.dtsi) and the carrier board
  .dtsi defined by this very patch, along with "model" and
  "compatible" properties.
The final DT architecture to describe the board is now:
r8a7743-iwg20d-q7.dts           # Carrier Board + SoM
├── r8a7743-iwg20m.dtsi         # SoM
│   └── r8a7743.dtsi            # SoC
└── iwg20d-q7-common.dtsi       # Carrier Board
and maximizes the reuse of the definitions for the carrier board
and for the SoM.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Chris Paterson <chris.paterson2@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>