PCI: designware: Consolidate outbound iATU programming functions

Currently, the outbound iATU programming functions are similar: the only
difference is index, type, addr and size.  Consolidate these functions into
one.  This saves about 1700 bytes in text:

   text	   data	    bss	    dec	    hex	filename
   9276	    204	      4	   9484	   250c pcie-designware.o-before
   7532	    204	      4	   7740	   1e3c pcie-designware.o

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Pratyush Anand <pratyush.anand@gmail.com>