of/flattree: Fix unhandled OF_DT_NOP tag when unflattening the device tree

NOPs within the property section are skipped, but NOPs between
OF_DT_END_NODE and OF_DT_BEGIN_NODE were not. My firmware NOPs out
entire nodes depending on various environment parameters.

of_scan_flat_dt already handles NOP more generally.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>