drivers: adc: rework Nordic SAADC driver

SHIM Nordic modification for the ADC driver controlling the SAADC
peripheral. Replaced HAL based implementation in favor of nrfx driver.

As a next step, it is planned to implement a feature that will allow
the peripheral SAADC timer to be used for sampling, and for this it is
necessary to use the nrfx driver in this SHIM.
This will allow more accurate and faster sampling than the kernel
mechanism currently provides.

Signed-off-by: Jakub Zymelka <jakub.zymelka@nordicsemi.no>