libfdt: Add fdt_setprop_empty()

Device trees can contain empty (zero length) properties, which are often
used as boolean flags.  These can already be created using fdt_setprop()
passing a length of zero and a pointer which is ignored.  It is safe to
pass NULL, but that may not be obvious from the interface.  To make it
clearer, add an fdt_setprop_empty() helper macro.

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