i2c: Make I2C available on UML

Remove the global dependency of the I2C subsystem on HAS_IOMEM and
move the dependency to the i2c/busses submenu, with an exception for
i2c-stub.

The generic I2C part does not need to have HAS_IOMEM set and thus now
becomes available in UML, so the I2C subsystem can now be used, e.g.
by the i2c-stub driver, for development of I2C device drivers.

[JD: Some adjustments.]

[Heiko Carstens: Keep I2C disabled on S390.]

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Jean Delvare <khali@linux-fr.org>