Correct ffs/fls regression for PowerPC etc

Commits

  02f99901
  52d61227

introduced a regression where platform-specific ffs/fls implementations
were defined away. This patch corrects that by using PLATFORM_xxx
instead of the name itself.

Signed-off-by: Simon Kagstrom <simon.kagstrom@netinsight.net>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Stefan Roese <sr@denx.de>