ata: start separating SATA specific code from libata-eh.c

Start separating SATA specific code from libata-eh.c:

* move sata_async_notification() to libata-sata.c:

Code size savings on m68k arch using (modified) atari_defconfig:

   text    data     bss     dec     hex filename
before:
  16243      18       0   16261    3f85 drivers/ata/libata-eh.o
after:
  16164      18       0   16182    3f36 drivers/ata/libata-eh.o

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>