nvme: Add shutdown function

Add a function to disable the NVMe controller. This will be used
to let the driver for the NVMe storage integrated on Apple SoCs
shutdown the NVMe controller such we can shutdown the NVMe
IOP controller in a clean way afterwards before handing control
to the OS.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested on: Macbook Air M1
Tested-by: Simon Glass <sjg@chromium.org>