ata: move sata_scr_*() to libata-sata.c

* move sata_scr_*() to libata-sata.c

* add static inlines for CONFIG_SATA_HOST=n case

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

   text    data     bss     dec     hex filename
before:
  35642     572      40   36254    8d9e drivers/ata/libata-core.o
  16607      18       0   16625    40f1 drivers/ata/libata-eh.o
after:
  32846     572      40   33458    82b2 drivers/ata/libata-core.o
  16243      18       0   16261    3f85 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>