ata: separate PATA timings code from libata-core.c

ata: separate PATA timings code from libata-core.c

Separate PATA timings code from libata-core.c:

* add PATA_TIMINGS config option and make corresponding PATA
  host drivers (and ATA ACPI code) select it

* move following PATA timings code to libata-pata-timings.c:
  - ata_timing_quantize()
  - ata_timing_merge()
  - ata_timing_find_mode()
  - ata_timing_compute()

* group above functions together in <linux/libata.h>

* include libata-pata-timings.c in the build when PATA_TIMINGS
  config option is enabled

* cover ata_timing_cycle2mode() with CONFIG_ATA_ACPI ifdef (it
  depends on code from libata-core.c and libata-pata-timings.c
  while its only user is ATA ACPI)

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

   text    data     bss     dec     hex filename
before:
  39688     573      40   40301    9d6d drivers/ata/libata-core.o
after:
  37820     572      40   38432    9620 drivers/ata/libata-core.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>