drivers: mfd: npm6001: initial version

Add an API-less MFD driver for nPM6001. In this case, the MFD device
driver doesn't expose any API as plain I2C API is used within other
device drivers (regulator, GPIO, watchdog). This driver just initializes
some device properties.

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