kern/91910: aac driver hangs on Dell PE800 with CERC SATA controller (regression from 5.4)

[Vivek Khera]

Ok, I finally did a bunch of more tests and it turns out that a 6.1- 
PRE kernel will boot if I disable ACPI timer with  
debug.acpi.disabled="timer" in /boot/loader.conf.

The system otherwise works except that the aac controller fails to  
complete shutdown (I filed another PR for that) resulting in  
potential data loss at reboot.