arm64: Do not rely on SPCR table to detect acpi

Since EDK2 commit d8e36289cef7bde628b023219cd65fa8e8d4562a, the Graphical console may
completely hide SPCR, causing panics later when locating timers.
As such simply rely on the ACPI Root pointer presence.

Submitted by: dan.kotowski@a9development.com
Reviewed by: andrew, mw
Differential Revision: https://reviews.freebsd.org/D27306