drivers: power: regulator: tps65941_regulator: Add support for 3Phase buck

Buck regulator 1, 2 and 3 of TPS6594132 on j721e-sk is in 3 Phase
confguration, in-order to support this, add configuring 3 Phase buck
in tps65941 while driver probing.

Signed-off-by: Sinthu Raja <sinthu.raja@ti.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>