nvme: apple: Add driver for Apple NVMe storage controller

Add a driver for the NVMe storage controller integrated on
Apple SoCs.  This NVMe controller isn't PCI based and deviates
from the NVMe standard in its implementation of the command
submission queue and the integration of an NVMMU that needs
to be managed.  This commit tweaks the core NVMe code to
support the linear command submission queue implemented by
this controller.  But setting up the submission queue and
managing the NVMMU controller is handled by implementing
the driver ops that were added in an earlier commit.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Tested-on: firefly-rk3399
Tested-by: Mark Kettenis <kettenis@openbsd.org>
Tested on: Macbook Air M1
Tested-by: Simon Glass <sjg@chromium.org>