i2c-iop3xx: Switch to static adapter numbering

Update the IOP3xx I2C driver to use i2c_add_numbered_adapter(), so that
later patches can convert boards to using new-style drivers.

Signed-off-by: Martin Michlmayr <tbm@cyrius.com>
Tested-by: Voipio Riku <Riku.Voipio@movial.fi>
Cc: Dan J Williams <dan.j.williams@intel.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>