adc: add microvolt conversion functions

Add a family of functions that convert to microvolts instead of
millivolts. The resolution of an ADC with a 600 mV reference and a 12
bit output (nRF SAADC for example) is an order of magnitude better than
millivolts (0.146 mV), even before considering non-unity gains.

Signed-off-by: Jordan Yates <jordan@embeint.com>