powerpc/CPM: Add i2c pins to dts and board setup

Initialize I2C pins on boards with CPM1/CPM2 controllers and document the
i2c bus in booting-without-of.

The boards don't have any I2C chips connected to the I2C bus, so unless
some external chips are connected to the boards, this code is just an
example of setting everything else up.

Signed-off-by: Jochen Friedrich <jochen@scram.de>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>