MAINTAINERS: Add myself as maintainer of the Ralink architecture

Its been a while since I am making contributions to this architecture.
Hence add myself as maintainer.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Acked-by: John Crispin <john@phrozen.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>