mesh: Add macros to encoded model publication periods

These macros are helpful for using the configuration client API with
periodic model publication.

nimble/host/mesh/include/mesh/cfg_cli.h

@@ -72,6 +72,48 @@ int bt_mesh_cfg_mod_app_bind_vnd(u16_t net_idx, u16_t addr, u16_t elem_addr,
 				 u16_t mod_app_idx, u16_t mod_id, u16_t cid,
 				 u8_t *status);
 
+/** @def BT_MESH_PUB_PERIOD_100MS
+ *
+ *  @brief Helper macro to encode model publication period in units of 100ms
+ *
+ *  @param steps Number of 100ms steps.
+ *
+ *  @return Encoded value that can be assigned to bt_mesh_cfg_mod_pub.period
+ */
+#define BT_MESH_PUB_PERIOD_100MS(steps)  ((steps) & BIT_MASK(6))
+
+/** @def BT_MESH_PUB_PERIOD_SEC
+ *
+ *  @brief Helper macro to encode model publication period in units of 1 second
+ *
+ *  @param steps Number of 1 second steps.
+ *
+ *  @return Encoded value that can be assigned to bt_mesh_cfg_mod_pub.period
+ */
+#define BT_MESH_PUB_PERIOD_SEC(steps)   (((steps) & BIT_MASK(6)) | (1 << 6))
+
+/** @def BT_MESH_PUB_PERIOD_10SEC
+ *
+ *  @brief Helper macro to encode model publication period in units of 10
+ *  seconds
+ *
+ *  @param steps Number of 10 second steps.
+ *
+ *  @return Encoded value that can be assigned to bt_mesh_cfg_mod_pub.period
+ */
+#define BT_MESH_PUB_PERIOD_10SEC(steps) (((steps) & BIT_MASK(6)) | (2 << 6))
+
+/** @def BT_MESH_PUB_PERIOD_10MIN
+ *
+ *  @brief Helper macro to encode model publication period in units of 10
+ *  minutes
+ *
+ *  @param steps Number of 10 minute steps.
+ *
+ *  @return Encoded value that can be assigned to bt_mesh_cfg_mod_pub.period
+ */
+#define BT_MESH_PUB_PERIOD_10MIN(steps) (((steps) & BIT_MASK(6)) | (3 << 6))
+
 struct bt_mesh_cfg_mod_pub {
 	u16_t  addr;
 	u16_t  app_idx;