MAINTAINERS: Add Shengjiu to reviewer list of sound/soc/fsl

Add Shengjiu who's actively working on the latest fsl/nxp audio drivers.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Shengjiu Wang <shengjiu.wang@gmail.com>
Link: https://lore.kernel.org/r/20200707045829.10002-1-nicoleotsuka@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>