samples: dmic: Add sample bit width configuration

- in order to support different bit widths introduce a Kconfig parameter
  SAMPLE_BIT_WIDTH used in the main.c - default is 16 bits
- for platforms with NXP micfil peripheral, default is 32 bits
- BYTES_PER_SAMPLE needs to correspond to BIT_WIDTH

Signed-off-by: Tomas Barak <tomas.barak@nxp.com>