docs: hwmon: Include 'xdpe12284.rst' into docs

Add documentation for 'xdpe122' devices.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Link: https://lore.kernel.org/r/20200113150841.17670-7-vadimp@mellanox.com


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