arm: sun7i: cubietruck: Enable the i2c controllers

The Cubietruck makes use of the first three i2c controllers found on the
Allwinner A20; i2c-0 is used internally for the PMIC, i2c-1 is exposed on
the board headers, and i2c-2 is used for DDC on the VGA connector. This
patch enables them in the device tree.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>