nvme: Get rid of the global variable nvme_info

At present the NVMe uclass driver uses a global variable nvme_info
to store global information like namespace id, and NVMe controller
driver's priv struct has a blk_dev_start that is used to calculate
the namespace id based on the global information from nvme_info.

This is not a good design in the DM world and can be replaced with
the following changes:

- Encode the namespace id in the NVMe block device name during
  the NVMe uclass post probe
- Extract the namespace id from the device name during the NVMe
  block device probe
- Let BLK uclass calculate the devnum for us by passing -1 to
  blk_create_devicef() as the devnum

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>