ARM: dts: r8a7791: add soc node

Add soc node to represent the bus and move all nodes with a base address
into this node. This is consistent with handling of R-Car Gen3, RZ/G1, and
R-Car V2H (R8A77920) SoCs upstream. It is intended to migrate other R-Car
Gen2 SoCs to this scheme.

The ordering is derived from simply moving each node with an address up to
before any nodes without a base address that occur before the soc node.  To
improve maintainability follow-up patches will sort subnodes of both the
new soc node and the root node.

This patch should not introduce any functional change.
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>