i2c: Make of_device_id array const

Make of_device_id array const, because all OF functions
handle it as const.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Peter Korsgaard <peter@korsgaard.com>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>