mfd: twl-core: Do not try to call legacy mfd add_children() when booted with DT

There is really no point to retry to add children devices in case the
of_platform_populate() fails.
We do not have any information provided via pdata in this case anyways.
Depending on the boot type (legacy or DT) only execute either one.

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