libfdt: Fix NOP handling bug in fdt_add_subnode_namelen()

libfdt: Fix NOP handling bug in fdt_add_subnode_namelen()

fdt_add_subnode_namelen() has a bug if asked to add a subnode to a
node which has NOP tags interspersed with its properties.  In this
case fdt_add_subnode_namelen() will put the new subnode before the
first NOP tag, even if there are properties after it, which will
result in an invalid blob.

This patch fixes the bug, and adds a testcase for it.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>