ARM: bootm: Try to use relocated ramdisk

After boot_ramdisk_high(), ramdisk would be relocated to
initrd_start & initrd_end, so use them instead of rd_start & rd_end.

Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>