drivers: mfd: add class skeleton

Introduce the skeleton for multi-function device drivers. This driver
class is inspired by the same class found in Linux. Multi-function
devices expose multiple functionalities, for example, a LED driver,
regulator, GPIO controller, etc. The MFD driver class serves as a kind
of "parent" device where common functionality can be managed: bus
access, IRQ sharing, initialization code, etc.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>