dt-bindings: i2c: Add Realtek RTL I2C Controller

Add dt-schema for the I2C controller on the RTL9300 Ethernet switch
with integrated SoC.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>