docs: hwmon: Document bel-pfe pmbus driver

Add documentation for bel-pfe pmbus driver which supports BEL PFE1100 and
PFE3000 power supplies.

Signed-off-by: Tao Ren <rentao.bupt@gmail.com>
Link: https://lore.kernel.org/r/20191029182054.32279-3-rentao.bupt@gmail.com


[groeck: Added bel-pfe to index.rst]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>