driver/audio: mpxxdtyy: Added support for 2 microphones (stereo)

In stereo case the pdm stream continuosly alternates 1-bit from
the left channel with 1-bit from the right channel. In this case
we need first to demultiplex channels bits on byte basis.
Then the Open_PDM_Filter library has to be called twice, one for
each channel.

Signed-off-by: Armando Visconti <armando.visconti@st.com>