i2c-piix4: Separate registration and probing code

Some chipsets have multiple sets of SMBus registers each controlling a
separate SMBus. Supporting these chipsets properly will require registering
multiple I2C adapters for one piix4.

The code to initialize and register the i2c_adapter structure has been
separated from piix4_probe and allows registration of a piix4 adapter
given its base address. Note that the i2c_adapter and i2c_piix4_adapdata
structures are now dynamically allocated.

Signed-off-by: Andrew Armenia <andrew@asquaredlabs.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>