mfd: Make MFD_AS3722 depend on I2C=y

MFD_AS3722 can only be builtin, so it needs I2C builtin as well.
With I2C=m, we get:

drivers/mfd/as3722.c:372: undefined reference to `devm_regmap_init_i2c'
drivers/built-in.o: In function `as3722_i2c_driver_init':
drivers/mfd/as3722.c:444: undefined reference to `i2c_register_driver'
drivers/built-in.o: In function `as3722_i2c_driver_exit':
drivers/mfd/as3722.c:444: undefined reference to `i2c_del_driver'

Signed-off-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Lee Jones <lee.jones@linaro.org>