i2c: rcar: implement atomic transfers

Implements atomic transfers. Tested by rebooting an r8a7790 Lager board
after connecting the i2c-rcar controller to the PMIC in
arch/arm/boot/dts/r8a7790-lager.dts like so:

		compatible = "i2c-demux-pinctrl";
		pinctrl-names = "default";
		pinctrl-0 = <&pmic_irq_pins>;
-		i2c-parent = <&iic3>, <&i2c3>;
+		i2c-parent = <&i2c3>, <&iic3>;
		i2c-bus-name = "i2c-pwr";
		#address-cells = <1>;
		#size-cells = <0>;

Signed-off-by: Ulrich Hecht <uli+renesas@fpond.eu>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Wolfram Sang <wsa@kernel.org>