Add support for a generic AHCI attachment. This allows us to attach to a
typically memory mapped bus, for example on the AMD Opteron A1100 the AHCI
device is mapped in the CPUs address space, and not through a PCI
controller.

Further work is needed for this to work with ACPI as this is expected to be
common on ARMv8 servers.

Reviewed by: mav, mmel
Obtained from: mmel, ABT Systems Ltd
Relnotes: yes
Sponsored by: SoftIron Inc
Differential Revision: https://reviews.freebsd.org/D4269