[POWERPC] Fix BootX booting with an initrd

The bootx_init.c trampoline didn't properly add the ramdisk to the
"reserve map" (list of reserved areas of memory), thus causing all sorts
of failures when using BootX with an initrd. Also fixes a possible
problem if the ramdisk is located before the device-tree passed by
BootX.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>