nvme: Add support for get-reg and set-reg commands

nvme: Add support for get-reg and set-reg commands

The get-reg command is to output register(s), it's possible to show
several registers when register name are used as command line option.

The set-reg command is to write nvme register(s). Again, the command is
able to set several registers when the register name are used.

Signed-off-by: Tokunori Ikegami <ikegami.t@gmail.com>