drivers: regulator: refactor PMIC binding to use physical PMIC IC

Refactor binding to use root PMIC IC, so that properties can be shared
between regulator devices. Each individual regulator output is still
created as an individual device, since the regulator API aligns with
these devices better than the PMIC IC itself.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>