units: Add SI metric prefix definitions

[ Upstream commit 26471d4a ]

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>
Stable-dep-of: c8c37bc5 ("i2c: designware: use casting of u64 in clock multiplication to avoid overflow")
Signed-off-by: Sasha Levin <sashal@kernel.org>