[POWERPC] U-boot passes the initrd as start/end, not start/size.

The boot wrapper platform init code on 83xx and 85xx using the cuboot
platform type was incorrectly assuming that u-boot supplied the size
of the initrd, whereas it actually supplies the end address.  This
fixes it.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>