powerpc/iseries: Fix unused function warning in iSeries DT code

If CONFIG_BLK_DEV_INITRD is unset dt_prop_u64() is unused, which
causes a warning. We don't really want to tie the definition to
BLK_DEV_INITRD, so mark it as maybe unused.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>