powerpc: Use correct type in prom_init.c

tce_entryp is a "u64 *" not an "unsigned long *".

[Split from a large patch -sfr]
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>