dt-bindings: can: rcar_can: document r8a774[35] can support

Document "renesas,can-r8a7743" and "renesas,can-r8a7745" compatible
strings. Since the fallback compatible string ("renesas,rcar-gen2-can")
activates the right code in the driver, no driver change is needed.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>