arm64: dts: renesas: r8a77995: Add CAN external clock support

Adds external CAN clock node for r8a77995. This clock can be used as
fCAN clock of CAN and CAN FD controller.

Based on a patch for r8a7796 by Chris Paterson.

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>