i2c: rcar_i2c: Add R-Car Gen4 support

Add support for R-Car Gen4 SoCs into the driver.

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,
which let us share the same driver.

Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Signed-off-by: Hai Pham <hai.pham.ud@renesas.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org> # Use RCAR_64 Kconfig
Reviewed-by: Heiko Schocher <hs@denx.de>