can: rcar_canfd: Abstract out DCFG address differences

Abstract the different addresses for the Channel n Data Bitrate
Configuration Register (DCFG) in the definition of the register macro,
like is already done for other register definitions, to simplify code
accessing this register.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/all/13e02d710dac3ddef73aa4be2b995766db9b6b4d.1674499048.git.geert+renesas@glider.be


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>