i2c: rcar: introduce Gen4 devices

So far, we treated Gen4 as Gen3. But we are soon adding FM+ as a Gen4
specific feature, so prepare the code for the new devtype.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>