ofwbus: remove arm64 ifdefs

Rather than using the DEVICE_IDENTIFY method, let's have other
ofwbus-using platforms add ofwbus0 explicitly in nexus, like arm64. This
gives them the same flexibility, e.g. if riscv starts supporting ACPI,
and cleans up the #ifdefs.

We were doing this already on riscv, but adjust the 'order' parameters.

Reviewed by: andrew, jhb
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D38492