Introduce new function OF_prop_free that frees 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 getprop functions' internals and leads to unneccessary code coupling

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