MAINTAINERS: mtd: add Brian Norris for MTD maintenance

While we're at it, change the git URL to remove the version legacy. It's
been a symlink to linux-mtd.git for a while now anyway.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: linux-mtd@lists.infradead.org
Acked-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>