MAINTAINERS: reflect actual changes in IEEE 802.15.4 maintainership

As the life flows, developers priorities shifts a bit. Reflect actual
changes in the maintainership of IEEE 802.15.4 code: Sergey mostly
stopped cared about this piece of code. Most of the work recently was
done by Alexander, so put him to the MAINTAINERS file to reflect his
status and to ease the life of respective patches.

Also add new net/mac802154/ directory to the list of maintained files.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Cc: Alexander Smirnov <alex.bluesman.smirnov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>