MAINTAINERS: add ATA sysfs file documentation to libata entry

Add the (still missing!) ATA sysfs file documentation to the libata
subsystem entry in the MAINTAINERS file.

Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>