dm: ofnode: rename ofnode_read_prop() to ofnode_get_property()

This function returns the pointer to the value of a node property.
The current name ofnode_read_prop() is confusing.  Follow the naming
of_get_property() from Linux.

The return type (const u32 *) is wrong.  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>