MAINTAINERS: Add entry for ARM

Since I've added various features in the arm architecture
support and review most of the patches nowadays, add myself
as a co-maintainer

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>