drivers: audio: dmic: Add support for multiple nrf PDM instances

Driver now uses multi-instance PDM nrfx API and
defines PDM device based on available instances.
It also introduces calculating PDM frequency using
prescaler, present on nRF54L15 FP1.
Updated nrfx API version changed to 3.7 to use the
new PDM API.

Signed-off-by: Michał Stasiak <michal.stasiak@nordicsemi.no>