Document ofw_bus_subr helpers "compatible" and "status" properties

Add documentation for following functions:

• ofw_bus_is_compatible
• ofw_bus_is_compatible_strict
• ofw_bus_node_is_compatible
• ofw_bus_search_compatible
• ofw_bus_get_status
• ofw_bus_status_okay
• ofw_bus_node_status_okay

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D14724