a newly introduced struct ofw_bus_devinfo which can hold the OFW info
of a device recallable via the ofw_bus KOBJ interface. Introduce a set
of functions ofw_bus_gen_get_*() which use ofw_bus_default_get_devinfo()
to provide generic subroutines for implementing the rest of the ofw_bus
KOBJ interface in a bus driver.
This is inspired by bus_get_resource_list() and bus_generic_rl_*_resource()
and allows to reduce code duplication in bus drivers as they only have
to provide an ofw_bus_default_get_devinfo() implementation in order to
provide the ofw_bus KOBJ interface via ofw_bus_gen_get_*().
- While here add a comment to ofw_bus_if.m describing the intention of
the ofw_bus KOBJ interface.
Reviewed by: marcel
