dt-bindings: mfd: lp87565: Add LP87561 configuration

lp87561 is a single output 4-phase regulator configuration.
Add support for the same.

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

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