mfd: twl-core: Convert to module_i2c_driver()

Shift TWL initialization to module/device init layer, because I2C now is
not initialized on subsys init layer and shifted to module/device init
layer instead.

The I2C <--> TWL dependency should be resolved in drivers/Makefile now.

Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>