MAINTAINERS: ARM/MStar/Sigmastar SoCs: Add a link to the MStar tree

Add a link to the MStar tree on github.

Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Reviewed-by: Romain Perier <romain.perier@gmail.com>
Signed-off-by: Olof Johansson <olof@lixom.net>