units: add conversion macros for percentage related units

units: add conversion macros for percentage related units

Add macros to convert between ratio and percentage related units,
including percent (1/100), permille (1/1,000), permyriad (1/10,000,
also equivalent to one Basis point) and per cent mille (1/100,000).
Those are  Used for precise fractional calculations in engineering,
finance, and measurement applications.

Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>