dm: ofnode: use fdt32_t for DT property value to fix sparse warning

DTB is encoded in big endian.  When we retrieve property values,
we need to use fdt32_to_cpu (aka be32_to_cpu) for endian conversion.
This is a bit error-prone, but sparse is useful to detect endian
mismatch.

We need to use (fdt32_t *) instead of (u32 *) for a pointer of a
property value.  Otherwise sparse warns "cast to restricted __be32".

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>