uart: uart_cpu_arm64: fix the build without FDT

clang 14 doesn't properly determine that we're unconditionally returning
if we have ACPI but not FDT. Push FDT setup entirely into a new
function, much like we currently do with ACPI, and just return ENXIO if
that doesn't succeed.

Reviewed by: andrew, manu (earlier version)
Differential Revision: https://reviews.freebsd.org/D36788