MAINTAINERS: blackfin: add git repository

Add the git repository currently in use for blackfin architecture
development.

This information was obtained from Steven Miao.

Signed-off-by: Michael Opdenacker <michael.opdenacker@free-electrons.com>
Cc: Steven Miao <realmz6@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>