MAINTAINERS: Add Spreadtrum SoC entry

Add Spreadtrum's maintainer entry and Orson, Baolin and me
as maintainers.

Cc: Orson Zhai <orsonzhai@gmail.com>
Cc: Baolin Wang <baolin.wang@linaro.org>

Signed-off-by: Chunyan Zhang <zhang.lyra@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>