shell: devmem: correct number of `mand` and `opt` of `cmd_dump`

According to the `SHELL_CMD_ARG` macro, the `mand` argument of `cmd_dump`
should be 5, including the command name itself, `dump`.
Meanwhile, the `opt` should be 2, as this value describes the optional
field, which is [-w <width>].

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>