of/flattree: Use common ALIGN() macro instead of arch specific _ALIGN

There's no reason to use the powerpc-specific _ALIGN macro in the fdt
code.  Replace it with ALIGN() from kernel.h

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-By: Jeremy Kerr <jeremy.kerr@canonical.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>