docs: hwmon: Add an entry for mp2888

The entry for mp2888 is missing and it causes the following
'make htmldocs' build warning:

Documentation/hwmon/mp2888.rst: WARNING: document isn't included in any toctree

Add the mp2888 entry.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20210521172218.37592-1-festevam@gmail.com


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