ARM: dts: r8a7792: add I2C support

Define the generic R8A7792 parts of the I2C[0-5] device nodes.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>