powerpc/powernv: Use uint64_t instead of size_t in OPAL APIs

Using size_t in our APIs is asking for trouble, especially
when some OPAL calls use size_t pointers.

Signed-off-by: Anton Blanchard <anton@samba.org>
Reviewed-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>