Maintainers.txt: add Masami as Socionext reviewer

Masami is the main contributor to these platforms, and would be
useful to have listed as a reviewer.

Signed-off-by: Leif Lindholm <leif@nuviainc.com>
Cc: Masami Hiramatsu <masami.hiramatsu@linaro.org>
Acked-by: Ard BIesheuvel <ardb@kernel.org>
Acked-by: Masami Hiramatsu <masami.hiramatsu@linaro.org>