nvme: LightNVM support

The first generation of Open-Channel SSDs is based on NVMe. The NVMe
driver is extended with support for the LightNVM command set.

Detection is made through PCI IDs. Current supported devices are the
qemu nvme simulator and CNEX Labs Westlake SSD. The qemu nvme enables
support through vendor specific bits in the namespace identification and
the CNEX Labs Westlake SSD implements a LightNVM compatible firmware and
is detected using the same method as qemu.

After detection, vendor specific codes are used to identify the device
and enumerate supported features.

Reviewed-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Javier González <jg@lightnvm.io>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>