mfd: tps6586x: move regulator dt parsing to regulator driver

Moving regulator node parsing to regulator driver in place
of parsing it on mfd driver.
The motivation for this change are:
- MFD core driver should not depends on regulator and able
  to instantiate device without regulator.
- The API for matching regulators are in regulator core and
  it is good that regulator driver only calls this API.
- Regulator specific support should be in regulator driver only
  to ease any enhancement/modification for regulators.
- The regulator driver is now registered as mfd sub device and
  all regulator registration is done from single probe call.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>