[libata sata_mv] SATA probe, DMA boundary fixes

- DMA boundary was being handled incorrectly.  Copied the code from
  ata_fill_sg(), since Marvell has the same DMA boundary needs.
  (we can't use ata_fill_sg directly since we have different hardware
   descriptors)
- cleaned up the SATA phy reset code, to deal with various errata