MAINTAINERS: Add entry for RDA Micro SoC architecture

Add MAINTAINERS entry for RDA Micro SoC architecture with myself
as the maintainer.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Olof Johansson <olof@lixom.net>