ata: add missing ATA_CMD_* defines

Add missing ATA_CMD_* defines to <linux/ata.h>.  Also add
ATA_EXABYTE_ENABLE_NEST, SETFEATURES_AAM_* and ATA_SMART_*
defines while at it.

Partially based on earlier work by Chris Wedgwood.

Acked-by: Chris Wedgwood <cw@f00f.org>
Acked-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>