i2c-eg20t: use i2c_add_numbered_adapter to get a fixed bus number

For EG20T and ML7213 IOH, the i2c controller numbers are fixed, using
fixed bus number will make it much easier for platform code to use
i2c_register_board_info() to register i2c devices.

Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>