MAINTAINERS: add Atheros maintainer for atlx

Jie Yang at Atheros is getting more directly involved with upstream work
on the atl* drivers.  This patch changes the ATL1 entry to ATLX (atl2
support posted to netdev today) and adds him as a maintainer.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>