drivers: sensor: ccs811: add interface for BASELINE management

Proper use of the CCS811 requires maintenance of the BASELINE
register value measured in clean air at various points in the sensor
life cycle.  The sensor driver abstraction has no facility to support
this, so add an application header with functions to fetch and update
the register value.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>