net: openthread: remove one subcommand level in shell

Currently OpenThread commands have to be prefixed in the shell by "ot
cmd", for example "ot cmd state". With the new shell, it is possible to
remove one subcommand level and change it to "ot state".

At the same time validate the number of arguments using
SHELL_CMD_ARG_REGISTER and improve the help message.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>