sys/phydat: Introduce PHYDAT_MIN, PHYDAT_MAX

sys/include/phydat.h

@@ -137,6 +137,16 @@ typedef struct {
     int8_t scale;               /**< the scale factor, 10^*scale* */
 } phydat_t;
 
+/**
+ * @brief   Minimum value for phydat_t::val
+ */
+#define PHYDAT_MIN  (INT16_MIN)
+
+/**
+ * @brief   Maximum value for phydat_t::val
+ */
+#define PHYDAT_MAX  (INT16_MAX)
+
 /**
  * @brief   Dump the given data container to STDIO
  *