post/lib_powerpc/multi.c: code cleanup

Clean up and document the code:

- get rid of unneeded code block
- add comment which code is generated

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Kumar Gala <galak@kernel.crashing.org>
Cc: Kim Phillips <kim.phillips@freescale.com>
Cc: Andy Fleming <afleming@gmail.com>
Acked-by: Anatolij Gustschin <agust@denx.de>