ahci: convert to use libata functions and definitions

libata already has similar functions as implemented in the ahci code.
Refactor the code to use the libata variants and remove the dependency on
ata.h. Convert some defines to use the version from libata.h. Also, remove
some unnecessary memset's of bss data.

This is a step toward hopefully merging ahci.c and dw_ahsata.c which are
essentially the same driver.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Reviewed-by: Tom Rini <trini@ti.com>