i2c: piix4: Export i2c_piix4 driver functions as library

Export the following i2c_piix4 driver functions as a library so that the
AMD ASF driver can utilize these core functionalities from the i2c_piix4
driver:

- piix4_sb800_region_request(): Request access to a specific SMBus region
on the SB800 chipset.

- piix4_sb800_region_release(): Release the previously requested SMBus
region on the SB800 chipset.

- piix4_transaction(): Handle SMBus transactions between the SMBus
controller and connected devices.

- piix4_sb800_port_sel(): Select the appropriate SMBus port on the SB800
chipset.

By making these functions available as a library, enable the AMD ASF
driver to leverage the established mechanisms in the i2c_piix4 driver,
promoting code reuse and consistency across different drivers.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Co-developed-by: Sanket Goswami <Sanket.Goswami@amd.com>
Signed-off-by: Sanket Goswami <Sanket.Goswami@amd.com>
Signed-off-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>