mfd: add 88pm88x driver

mfd: add 88pm88x driver

Marvell 88PM880 and 8PM886 are two similar PMICs with mostly matching
register mapping. They provide various functions such as onkey, battery,
charger and regulators.

Add support for 88PM886 found for instance in the samsung,coreprimevelte
smartphone with which this was tested. Support for 88PM880 is not
implemented here but should be straightforward to add.

Implement only the most basic support omitting the currently unused
registers and I2C subclients which should thus be added with the
respective subdevices. However, add support for the onkey already.

Signed-off-by: Karel Balej <balejk@matfyz.cz>