Add OF_prop_free function as a counterpart for OF_*prop_alloc

• Introduce new OF API function OF_prop_free to free memory allocated by OF_getprop_alloc and OF_getencprop_alloc. Current code just calls free(9) with M_OFWPROP memory class which assumes knowledge about OF_*prop_alloc functions' internals and leads to unneccessary code coupling

• Convert some of the free(..., M_OFWPROP) instances to OF_prop_free

Files affected by this commit are the ones I was able to test on real
hardware. The rest of free(..., M_OFWPROP) instances will be handled with
idividual maintainers

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D6315