Expand OpenFirmware API with ofw_bus_node_status_okay
method

Method could be used before we can access device_t structure.
As per simple phandle_t handle we can access FDT to check
if specified node has been enabled.
It will be used in Marvell's common configuration code.

Submitted by: Konrad Adamczyk <ka@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Reviewed by: zbb, meloun-miracle-cz
Differential revision: https://reviews.freebsd.org/D9218