dm: core: Don't include ofnode functions with of-platdata

These functions cannot work with of-platdata since libfdt is not
available. At present when dev_read_...() functions are used it produces
error messages about ofnode which is confusing.

Adjust the Makefile and header to produce an error message for the actual
dev_read...() function which is called. This makes it easier to see what
code needs to be converted for use with of-platdata.

Signed-off-by: Simon Glass <sjg@chromium.org>