net: pse-pd: Add Si3474 PSE controller driver

net: pse-pd: Add Si3474 PSE controller driver

Add a driver for the Skyworks Si3474 I2C Power Sourcing Equipment
controller.

Driver supports basic features of Si3474 IC:
- get port status,
- get port power,
- get port voltage,
- enable/disable port power.

Only 4p configurations are supported at this moment.

Signed-off-by: Piotr Kubik <piotr.kubik@adtran.com>
Reviewed-by: Kory Maincent <kory.maincent@bootlin.com>
Link: https://patch.msgid.link/9b72c8cd-c8d3-4053-9c80-671b9481d166@adtran.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>