MAINTAINERS: add entry for Sound Open Firmware drivers

MAINTAINERS: add entry for Sound Open Firmware drivers

Sound Open Firmware (SOF) is an open source audio DSP firwmare
instrastructure and SDK. The kernel drivers for SOF are part of
the ALSA subsystem.

Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Acked-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Acked-by: Liam Girdwood <liam.r.girdwood@linux.intel.com>
Acked-by: Daniel Baluta <daniel.baluta@nxp.com>
Acked-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200311174537.24497-1-kai.vehmanen@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>