ARM: shmobile: r8a7794: add I2C DT support

Define the generic R8A7794 parts of the I2C[0-5] device nodes.

Based on the original patch by Koji Matsuoka <koji.matsuoka.xm@renesas.com>.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>