regulator: tps6594-regulator: Correct multi-phase configuration

commit 31856b260fb9c1e7519606604d353f3e41949508 from
git://git.ti.com/ti-linux-kernel/ti-linux-kernel.git

commit 74b38cd7 upstream

According to the TPS6594 PMIC Manual (linked) 8.3.2.1.4 Multi-Phase BUCK
Regulator Configurations section, the PMIC ignores all the other bucks'
except the primary buck's regulator registers. This is BUCK1 for
configurations BUCK12, BUCK123 and BUCK1234 while it is BUCK3 for
BUCK34. Correct the registers mapped for these configurations
accordingly.

Fixes: f17ccc5d

 ("regulator: tps6594-regulator: Add driver for TI TPS6594 regulators")
Link: https://www.ti.com/lit/gpn/tps6594-q1
Signed-off-by: Neha Malcom Francis <n-francis@ti.com>
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>