compal-laptop: Add charge control limit

Add charge control limit to the power supply subsystem of the Compal
platform driver.

This apparently was present in the original driver by Cezary Jackiewicz
at http://eko.one.pl/index.php?page=compal-laptop

 but it seems to have been
overlooked.

The Kconfig description is updated to reflect this addition. It now also
mentions the hwmon interface that was already present.

Signed-off-by: Roald Frederickx <roald.frederickx@gmail.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>