SPINEL_CMD_PEEK value

Peek command (Host -> NCP) Encoding: `LU` `L` : The address to peek `U` : Number of bytes to read This command allows the NCP to fetch values from the RAM of the NCP for debugging purposes. Upon success, `CMD_PEEK_RET` is sent from the NCP to the host. Upon failure, `PROP_LAST_STATUS` is emitted with the appropriate error indication. The NCP MAY prevent certain regions of memory from being accessed. This command requires the capability `CAP_PEEK_POKE` to be present.