iio: Add some helper macros for unit conversion

Some datasheets use a different unit to specify the channel scale than what IIO
expects it to be. This patch adds two helper macros which allow to convert units
commonly used in datasheets to IIO units:

 * acceleration: g -> meter / second**2
 * angular velocity: degree (/ second) -> rad (/ second)

This makes it much more convenient to specify and also easier to verify a
channel's scale attribute.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>