units: complement the set of Hz units

Currently, Hz units do not have milli, micro and nano Hz coefficients.
Some drivers (IIO especially) use their analogues to calculate
appropriate Hz values. This patch includes them to units.h definitions,
so they can be used from different kernel places.

Signed-off-by: Dmitry Rokosov <ddrokosov@sberdevices.ru>
Link: https://lore.kernel.org/r/20220812165243.22177-3-ddrokosov@sberdevices.ru


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>