PPC: 85xx: Generalize DDR TLB mapping function

The DDR mapping function really is just a generic virtual -> physical
mapping function. Generalize it so it can support any virtual starting
offset and IO maps just the same.

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Scott Wood <scottwood@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>