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>

arch/powerpc/platforms/iseries/dt.c

@@ -204,7 +204,8 @@ static void __init dt_prop_u32(struct iseries_flat_dt *dt, const char *name,
 	dt_prop(dt, name, &data, sizeof(u32));
 }
 
-static void __init dt_prop_u64(struct iseries_flat_dt *dt, const char *name,
+static void __init __maybe_unused dt_prop_u64(struct iseries_flat_dt *dt,
+					      const char *name,
 		u64 data)
 {
 	dt_prop(dt, name, &data, sizeof(u64));