i2c: Only build Tyan SMBus mux drivers on x86

The two Tyan SMBus mux drivers (i2c-amd756-s4882 and i2c-nforce2-s4985)
are only useful on specific x86 motherboards, so there is no point in
letting them be built on other architectures.

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