can: rcar_canfd: Add Classical CAN only mode support

The controller can operate in one of the two global modes
 - CAN FD only mode (default)
 - Classical CAN (CAN2.0) only mode

This patch adds support for Classical CAN only mode. It can be enabled
by defining the optional device tree property "renesas,no-can-fd" of this
node.

Note: R-Car Gen3 h/w manual v0.51E shows bit6 of RSCFDnCFDGCFG as
reserved, which is incorrect. This bit is same as RSCFDnGCFG.

Signed-off-by: Ramesh Shanmugasundaram <ramesh.shanmugasundaram@bp.renesas.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>