i2c-piix4: Add support to SB800 SMBus changes

Add support for the AMD SB800 Family series of products.
Major changes include the changes to addressing the SMBus registers at different
location from the locations in the previous compatible parts from AMD such as
SB400/SB600/SB700. For SB800, the main features and register definitions of
SMBus and other interfaces are still compatible with the previous products with
the only change being in how to access the internal registers for these blocks.

Signed-off-by: Shane Huang <shane.huang@amd.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>