ofwbus: remove handling of resources from ofwbus

The architecture nexus should handle allocation and release of memory and
interrupts. This is to ensure that system-wide resources such as these
are available to all devices, not just children of ofwbus0.

On powerpc this moves the ownership of these resources up one level,
from ofwbus0 to nexus0. Other architectures already have the required
logic in their nexus implementation, so this eliminates the duplication
of resources. An implementation of nexus_adjust_resource() is added for
arm, arm64, and riscv.

As noted by ian@ in the review, resource handling was the main bit of
logic distinguishing ofwbus from simplebus. With some attention to
detail, it should be possible to merge the two in the future.

Co-authored by: mhorne
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D30554