fdt: ppc: Drop extra size for ramdisk

This code dates from around 2008:

   56844a22 powerpc: Fix bootm to boot up again with a Ramdisk

Since then we have added FDT relocation which provides enough space
for expansion. We have also added all sorts of fixups earlier in
image_setup_libfdt() which require more space, with ramdisk being the
least of them.

Therefore this extra hack for ramdisk seems unnecessary. Drop it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>