[man] Add documentation for OpenFirmware API

Add man pages for following OpenFirmware functions:

OF_child, OF_parent, OF_peer, OF_device_from_xref,
OF_device_register_xref, OF_xref_from_device, OF_getprop,
OF_getencprop, OF_getencprop_alloc, OF_getprop_alloc, OF_getproplen,
OF_hasprop, OF_nextprop, OF_prop_free, OF_searchencprop,
OF_searchprop, OF_setprop

Edited by: wblock
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D14511