device_shutdown() called from reboot or power_shutdown expect
all devices to be shutdown. Same is true for even ahci pci driver.
As no ahci shutdown function is implemented, the ata subsystem
always remains alive with DMA & interrupt support. File system
related calls should not be honored after device_shutdown().

So defining ahci pci driver shutdown to freeze hardware (mask
interrupt, stop DMA engine and free DMA resources).
Signed-off-by: NPrabhakar Kushwaha <pkushwaha@marvell.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>