hwmon: add Corsair PSU HID controller driver

The Corsair digital power supplies of the series RMi, HXi and AXi include
a small micro-controller with a lot of sensors attached. The sensors can
be accessed by an USB connector from the outside.

This micro-controller provides the data by a simple proprietary USB HID
protocol. The data consist of temperatures, current and voltage levels,
power usage, uptimes, fan speed and some more. It is also possible to
configure the PSU (fan mode, mono/multi-rail, over current protection).

This driver provides access to the sensors/statistics of the RMi and HXi
series power supplies. It does not support configuring these devices,
because there would be many ways to misconfigure or even damage the PSU.

This patch adds:
- hwmon driver corsair-psu
- hwmon documentation
- updates MAINTAINERS

Signed-off-by: Wilken Gottwalt <wilken.gottwalt@posteo.net>
Link: https://lore.kernel.org/r/20201027131710.GA253280@monster.powergraphx.local


Signed-off-by: Guenter Roeck <linux@roeck-us.net>