arm64: dts: r8a7796: Add I2C for DVFS device node

This patch adds I2C for DVFS device node for R8A7796 SoC.

Signed-off-by: Dien Pham <dien.pham.ry@rvc.renesas.com>
Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>