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>

include/drivers/sensor/ccs811.h

@@ -0,0 +1,58 @@
+/*
+ * Copyright (c) 2018 Peter Bigot Consulting, LLC
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @file
+ * @brief Extended public API for CCS811 Indoor Air Quality Sensor
+ *
+ * Some capabilities and operational requirements for this sensor
+ * cannot be expressed within the sensor driver abstraction.
+ */
+
+#ifndef ZEPHYR_INCLUDE_DRIVERS_SENSOR_CCS811_H_
+#define ZEPHYR_INCLUDE_DRIVERS_SENSOR_CCS811_H_
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <device.h>
+
+/**
+ * @brief Fetch the current value of the BASELINE register.
+ *
+ * The BASELINE register encodes data used to correct sensor readings
+ * based on individual device configuration and variation over time.
+ *
+ * For proper management of the BASELINE register see AN000370
+ * "Baseline Save and Restore on CCS811".
+ *
+ * @param dev Pointer to the sensor device
+ *
+ * @return a non-negative 16-bit register value, or a negative errno
+ * code on failure.
+ */
+int ccs811_baseline_fetch(struct device *dev);
+
+/**
+ * @brief Update the BASELINE register.
+ *
+ * For proper management of the BASELINE register see AN000370
+ * "Baseline Save and Restore on CCS811".
+ *
+ * @param dev Pointer to the sensor device
+ *
+ * @param baseline the value to be stored in the BASELINE register.
+ *
+ * @return 0 if successful, negative errno code if failure.
+ */
+int ccs811_baseline_update(struct device *dev, u16_t baseline);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* ZEPHYR_INCLUDE_DRIVERS_SENSOR_CCS811_H_ */