hwmon: (amd_energy) Add documentation

Document amd_energy driver with energy sensors supported by it.

Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Naveen Krishna Chatradhi <nchatrad@amd.com>
Link: https://lore.kernel.org/r/20200519155011.56184-2-nchatrad@amd.com


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