dt-bindings: i2c: amlogic,meson6-i2c: add optional power-domains

On newer SoCs, the I2C hardware can require a power domain to operate.
Since the same compatible is used for older and newer SoCs make
power-domains property optional.

Signed-off-by: George Stark <gnstark@salutedevices.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>