mfd: Allow i2c modular drivers to build with I2C=m

These drivers can be built as module, so make them depend on I2C rather
than I2C=y.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Adam Thomson <Adam.Thomson.Opensource@diasemi.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>