i2c-piix4: Blacklist two mainboards

We had a report that running sensors-detect on a Sapphire AM2RD790
motherbord killed the CPU. While the exact cause is still unknown,
I'd rather play it safe and prevent any access to the SMBus on that
machine by not letting the i2c-piix4 driver attach to the SMBus host
device on that machine. Also blacklist a similar board made by DFI.

Signed-off-by: Jean Delvare <khali@linux-fr.org>