units: Add SI metric prefix definitions

Sometimes it's useful to have well-defined SI metric prefix to be used
to self-describe the formulas or equations.

List most popular ones in the units.h.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

include/linux/units.h

@@ -4,6 +4,22 @@
 
 #include <linux/math.h>
 
+/* Metric prefixes in accordance with Système international (d'unités) */
+#define PETA	1000000000000000ULL
+#define TERA	1000000000000ULL
+#define GIGA	1000000000UL
+#define MEGA	1000000UL
+#define KILO	1000UL
+#define HECTO	100UL
+#define DECA	10UL
+#define DECI	10UL
+#define CENTI	100UL
+#define MILLI	1000UL
+#define MICRO	1000000UL
+#define NANO	1000000000UL
+#define PICO	1000000000000ULL
+#define FEMTO	1000000000000000ULL
+
 #define MILLIWATT_PER_WATT	1000L
 #define MICROWATT_PER_MILLIWATT	1000L
 #define MICROWATT_PER_WATT	1000000L