can: rcar_canfd: Add support for R-Car Gen4

Despite the name, R-Car V3U (R8A779A0) was the first member of the R-Car
Gen4 family.  Generalize the support for R-Car V3U to other SoCs in the
R-Car Gen4 family by adding a family-specific compatible value, and by
replacing all references to "V3U" by "Gen4".
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/all/61f6f34eb7bcc62ff604add98f1bcd2d2584187d.1674499048.git.geert+renesas@glider.beSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>