arm64: dts: r8a7796: Use R-Car Gen 3 fallback binding for i2c nodes

Use recently added R-Car Gen 3 fallback binding for i2c nodes in
DT for r8a7796 SoC.

This has no run-time effect for the current driver as the initialisation
sequence is the same for the SoC-specific binding for r8a7796 and the
fallback binding for R-Car Gen 3.

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