i2c: piix4: Support alternative port selection register

The SB800 register reference guide says that the SMBus port selection
bits may not always be in register Smbus0En (0x2c) but could
alternatively be found in register Smbus0Sel (0x2e) depending on the
settings in register Smbus0SelEn (0x2f.) Add support for this
configuration.

The "alternative" register is the only one working for the Bolton
(aka Hudson-2) chipset anyway. I do not have any documentation for
the "kerncz" chipset so we treat it the same as the Bolton for now.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Tested-by: Christian Fetzer <fetzer.ch@gmail.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>