shell: devmem: minor optimizations

- Use `SHELL_CMD_ARG_REGISTER` for the main cmd to state the
  required number of arguments, this helps to remove the
  runtime check from the command, and also print the help
  message to the terminal when the argument count is
  unexpected.
- Some changes to the help text that hopefully makes the
  mandatory and optional arguments more obvious to the user

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>
(cherry picked from commit 276ccd04)