can: rcar_can: Remove unused platform data support

All R-Car platforms use DT for describing CAN controllers. R-Car CAN
platform data support was never used in any upstream kernel.

Move the Clock Select Register settings enum into the driver, and remove
platform data support and the corresponding header file.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>