dm: core: Allow copying ofnode property data when writing

At present ofnode_write_prop() is inconsistent between livetree and
flattree, in that livetree requires the caller to ensure the property
value is stable (e.g. in rodata or allocated) but flattree does not, since
it makes a copy.

This makes the API call a bit painful to use, since the caller must do
different things depending on OF_LIVE.

Add a new 'copy' argument which tells the function to make a copy if
needed. Add some tests to cover this behaviour.

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