nvme: Add nvme commands

Add nvme commands in U-Boot command line.

1. "nvme scan" - scan NVMe blk devices
2. "nvme list" - show all available NVMe blk devices
3. "nvme info" - show current or a specific NVMe blk device
4. "nvme device" - show or set current device
5. "nvme part" - print partition table
6. "nvme read" - read data from NVMe blk device
7. "nvme write" - write data to NVMe blk device

Signed-off-by: Zhikang Zhang <zhikang.zhang@nxp.com>
Signed-off-by: Wenbin Song <wenbin.song@nxp.com>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>