nvme: Introduce driver ops

The NVMe storage controller integrated on Apple SoCs deviates
from the NVMe standard in two aspects.  It uses a "linear"
submission queue and it integrates an NVMMU that needs to be
programmed for each NVMe command.  Introduce driver ops such
that we can set up the linear submission queue and program the
NVMMU in the driver for this strange beast.

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>