arm64: Switch to ACPI by default

When the FreeBSD/arm64 port was created we only supported FDT. We now
also support ACPI, and have for many years. When this support was
added we kept FDT as the default.

There are some setups where both ACPI tables and a FDT DTB are passed
into the kernel. In most of these cases the DTB is only used to pass
in minimal information.

To handle the cases where both are passed in prefer ACPI over FDT.

Reviewed by: bz, imp, emaste
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D46750