bootm, linux: fix booting Multi-File Image with "kernel+ramdisk+fdt"

Booting a "Multi-File Image" including a linux kernel, ramdisk and
fdt, generated with

mkimage -A ppc \
    -O linux \
    -T multi \
    -C gzip \
    -a 00000000 \
    -e 00000000 \
    -n "kernel-2.6+initrd+dtb" \
    -d "vmlinux.bin.gz:ramdisk_image.gz:board.dtb" \
    multi.bin

actually fails, because ramdisk start and end addresses
didn;t get initialized. This patch fixes this issue.

Tested on the KUP4K board.

Signed-off-by: Heiko Schocher <hs@denx.de>