Document IACx/DACx registers access using ONE_REG API

Patch to access the debug registers (IACx/DACx) using ONE_REG api
was sent earlier. But that missed the respective documentation.

Also corrected the index number referencing in section 4.69

Signed-off-by: Bharat Bhushan <bharat.bhushan@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>