arm64: dts: r8a7795: Add CAN external clock support

Adds external CAN clock node for r8a7795. This clock can be used as
fCAN clock of CAN and CAN FD controller.

Signed-off-by: Ramesh Shanmugasundaram <ramesh.shanmugasundaram@bp.renesas.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>