i2c-viapro: Add VX800/VX820 support

Thanks to new datasheets published on http://linux.via.com.tw we can now add 
support for VX800/VX820 chipsets.

Signed-off-by: Rudolf Marek <r.marek@assembler.cz>
Signed-off-by: Jean Delvare <khali@linux-fr.org>