drivers: adc: Introduce reworked API

This commit replaces the API for ADC drivers with a reworked one.
As requested in the issue #3980, the adc_enable/adc_disable functions
are removed. Additionaly, some new features are introduced, like:
- asynchronous calls
- configuration of channels
- multi-channel sampling

Common parts of code that are supposed to appear in each implementation
of the driver (like locking, synchronization, triggering of consecutive
samplings) are provided in the "adc_context.h" file to keep consistency
with the SPI driver. Syscalls are no longer present in the API because
the functions starting read requests cannot use them, since they can be
provided with a callback that is executed in the ISR context, and there
is no point in supporting syscalls only for the channels configuration.

"adc_api" test is updated and extended with additional test cases,
with intention to show how the API is supposed to be used.
"adc_simple" test is removed as it does not seem to add much value.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>