MAINTAINERS: Add Niklas Cassel as libata maintainer

Add Niklas as co-maintainer of the ata subsystem (aka libata). The new
shared tree for libata will now be
"pub/scm/linux/kernel/git/libata/linux" on kernel.org GIT.

Niklas and I will alternate maintainership every release.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Acked-by: Niklas Cassel <cassel@kernel.org>