regulator: tps6594-regulator: Correct multi-phase configuration

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>
Link: https://msgid.link/r/20240521094758.2190331-1-n-francis@ti.com
Signed-off-by: Mark Brown <broonie@kernel.org>