ext: Added OpenPDM2PCM ST external library

This library, written by STMicroelectronics, is used to convert an
audio stream from PDM format to PCM format through a signal filtering
and decimation.

Library is located in ext/hal/st/lib/audio.

Origin: ST Microelectronics
License: Apache 2.0
URL: https://os.mbed.com/


Commit: 25:f2c04f757003
Purpose: reconstruct the audio signal produced by ST MEMS microphone
Maintained-by: External

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