dm: ofnode: change return type of dev_read_prop() to opaque pointer

DT property values can be strings as well as integers.  This is why
of_get_property/fdt_getprop returns an opaque pointer.

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