regulator: lp87565: Add 4-phase lp87561 regulator support

The LP8756x family has a single output 4-phase regulator
configuration. Add support for the same. The control
lies in the master buck which is buck0 for 4-phase
configuration. Enable/disable/voltage set happen via
buck0 registers.

Data Sheet: https://www.ti.com/lit/ds/symlink/lp87561-q1.pdf

Signed-off-by: Keerthy <j-keerthy@ti.com>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>