The Arm pl011 driver assumes it's running a devicetree based system.
It calls OF_* functions to check if it needs to implement workarounds.
This may not be the case on arm64 where we support both FDT and ACPI.
Fix this by checking if we are booting on FDT before calling these checks.

Reviewed by: ian
Sponsored by: DARPA, AFRL
Sponsored by: Cavium (Hardware)
Differential Revision: https://reviews.freebsd.org/D14515