add a sysctl to the AHCI driver to allow disabling the PHY

[chs]

Summary

As part of my work on fixing kernel crashes caused by disk I/O errors, it is useful to be able to trigger various kinds of errors. This patch allows causing an AHCI-attached disk to disappear, by having the driver keep the PHY disabled when the driver would otherwise enable the PHY. It also allows making the disk reappear by having the driver go back to setting the PHY enable/disable state as it normal would and simulating the hardware event that causes a bus rescan.

Test Plan

I've verified that toggling this new sysctl makes the corresponding disk disappear or reappear, and that CAM reacts to these events as expected.

[imp]

This looks good to my eye. I'll rebase and commit after a quick test unless I hear otherwise....