pylibfdt: Return string instead of bytearray from getprop()

The return value is not actually mutable, so it seems more correct to
return bytes rather than a bytearray.

Suggested-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>