[POWERPC] Fix zImage.coff generation for 32-bit pmac

Commit 9da82a6dee9db4cd5ae7a74ab4f51afb52b6efb9 inadvertently
removed the platform override for zImage.coff to be generated
with pmaccoff.   Rather than add a special makefile rule,
change the platform for which the wrapper platform uses
the special rules.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/boot/wrapper

@@ -129,7 +129,7 @@ case "$platform" in
 pmac|pseries|chrp)
     platformo=$object/of.o
     ;;
-pmaccoff)
+coff)
     platformo=$object/of.o
     lds=$object/zImage.coff.lds
     ;;
@@ -220,7 +220,7 @@ case "$platform" in
 pseries|chrp)
     $object/addnote "$ofile"
     ;;
-pmaccoff)
+coff)
     ${CROSS}objcopy -O aixcoff-rs6000 --set-start "$entry" "$ofile"
     $object/hack-coff "$ofile"
     ;;