MAINTAINERS: name the libata/linux for-next branch

The T: entry for LIBATA SUBSYSTEM (Serial and Parallel ATA drivers)
names libata/linux without a branch.  The repository's HEAD pointer
points to branch master, which has no active development.  Active
development is on the for-next branch.  Name the branch so the entry
identifies where development happens.

Documentation/process/submitting-patches.rst sends contributors to the
T: entry to find the tree to prepare patches against, so a branch-less
entry whose HEAD is already in mainline points them to the wrong branch.

Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Matthias Goergens <matthias.goergens@gmail.com>
Link: https://lore.kernel.org/r/20260925052329.2683619-1-matthias.goergens@gmail.com
Signed-off-by: Niklas Cassel <cassel@kernel.org>