doc:power:pmic: Add doc entry for PMIC(v2) framework

Well written documentation for PMIC framework was missing and hence
it has been probably difficult and time consuming for other developers
to understand rationale for key design decisions and overall design
structure.
This commit provides proper documentation entry.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>