mfd: simple-mfd-i2c: Add configuration for RPi POE HAT

The Raspbery Pi PoE+ HAT exposes a fan controller and power
supply status reporting via a single I2C address.

Create an MFD device that allows loading of the relevant
sub-drivers, with a shared I2C regmap.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>