powerpc: Remove bi_memstart & bi_memsize assignments in spl.c

Most likely these deprecated (removed) variables are not needed. Lets
remove the assignments completely from all spl.c files.

Signed-off-by: Stefan Roese <sr@denx.de>
Tested-by: Oleksandr Zhadan and Michael Durrant
Reviewed-by: Simon Glass <sjg@chromium.org>