mfd: twl-core: Clean up and correct child registration

Make the twl child registration calls a bit more uniform by always using the
SUB_CHIP_ID* define instead of the mixed use of the define and magic number.

At the same time correct the following devices so they are registered for the
correct parent device (i2c slave):
twl4030_wdt is accessible on 0x4b address and not 0x48
twl4030_pwrbutton is accessible on 0x4b address and not 0x49
twl4030-audio is on 0x49 all the time

Acked-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>