drivers: adc: Add shims for nrfx ADC and SAADC drivers

This commit adds translation layers to make nrfx drivers for the nRF
ADC (nRF51 series) and SAADC (nRF52 series) peripherals accessible via
the Zephyr's API. The SAADC peripheral is accessed using nrfx HAL only
as it turns out that usage of the nrfx driver in this case would be
inconvenient and would unnecessarily complicate the shim.

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