adc: Convert mcux adc16 driver to the new adc api

Major rework of the mcux adc16 driver to convert it to the new adc api.
Currently supports a subset of the api features including synchronous
and asynchronous reads, and consecutive reads triggered by the kernel
timer.

Does not yet support some of the channel configuration options such as
gain and reference voltage because the hardware only allows these
options to be configured by peripheral instance rather than by channel.
The values are currently hardcoded in the driver, but in the future we
could introduce some flexibility per instance via device tree
attributes.

Does not yet support consecutive reads triggered by a hardware timer.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>