Add CAN nodes to r8a7794 device tree.

Based on work by Sergei Shtylyov for the r8a7791 SoC.
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Ramesh Shanmugasundaram <ramesh.shanmugasundaram@bp.renesas.com>