powerpc/bootm: Flush ramdisk and device tree image when booting on MP

We already flush the kernel image after we've loaded it to ensure
visiblity to the other cores.  We need to do the same thing for the
ramdisk and device tree images.  In AMP boot scenarios we might not be
HW cache coherent with the secondary core that we are loading and
setting the ramdisk and device tree up for.  Thus we need to ensure
we've flushed the regions of memory utilized by ramdisk and device tree
so the loadding and any modifications (from decompression or fdt updates)
are made visible to the secondary cores.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>