hwmon: Add support for the sl28cpld hardware monitoring controller

Add support for the hardware monitoring controller of the sl28cpld board
management controller. This driver is part of a multi-function device.

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>