ahci(4): Allow enclosure emulation without hardware.

After 53f5ac1310e allowed SATA device mapping to enclosure slots,
it may have sense to provide enclosure device emulation even without
real hardware interface like SGPIO just for purposes of physical
device location tracking (still assuming straight cabling).

MFC after: 1 week
Sponsored by: iXsystems, Inc.

(cherry picked from commit 9aba757e92aaf0751c83c0ce3f18f65f864f1811)