dt-bindings: i2c: nomadik: Add power domain to binding

The power domain is used in the example and in the device trees
for this IP block. Add it.

Reported-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>