MAINTAINERS: add Masahiro Yamada as a Kbuild maintainer

It has been difficult lately for Michal to work on Kbuild on his
regular basis.  We discussed the maintainership of Kbuild, and I
decided to be a co-maintainer.

Add myself to the maintainer field, and replace the repository with
my own.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Michal Marek <mmarek@suse.com>