arm64: dts: r8a7796: Add CAN external clock support

Adds external CAN clock node for r8a7796. This clock can be used as
fCAN clock of CAN and CAN FD controller.

Based on a patch for r8a7795 by Ramesh Shanmugasundaram.

Signed-off-by: Chris Paterson <chris.paterson2@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>