ahci: Separate the AHCI state structure into the header

ahci: Separate the AHCI state structure into the header

Pull the AHCI state structure out into the header. This allows
other containers to access the struct. This is required to add
the device to modern SoC containers.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Sai Pavan Boddu <saipava@xilinx.com>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>