MAINTAINERS: Add Alper as a binman maintainer

I ended up learning most of binman internals while trying to add a few
features to it, and I recently started reviewing binman series that
would not affect me personally. I'll keep working on it and try to do
more reviews.

Add myself as a maintainer for binman.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Acked-by: Tom Rini <trini@konsulko.com>