i2c: rcar: Add R-Car Gen4 support

Add support for the I2C Bus Interface on R-Car Gen4 SoCs (e.g. R-Car
S4-8) by matching on a family-specific compatible value.

While I2C on R-Car Gen4 does support some extra features (Slave Clock
Stretch Select), for now it is treated the same as I2C on R-Car Gen3.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
[wsa: removed incorrect "FM+" from commit message]
Signed-off-by: Wolfram Sang <wsa@kernel.org>