ARM: dts: r8a7794: Add IIC nodes

Add IIC nodes to r8a7794 device tree.

Based on similar work for the r8a7793 by Laurent Pinchart.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>