add minimum-nominal-power to rtpc and consolidate nominal-limits in a grouping

[mamali68]

Since minimum-nominal-power is a system capability of the radio it is needed in case of remote control as well, to maintain backward-compatibility, the suggestion is that it should be added to container rtpc as well.

Secondly, raising the proposal to create a grouping to consolidate both attributes.

[...]
grouping nominal-power-limits {
    description
      "The minimum and maximum power capabilities of the radio";
        leaf maximum-nominal-power {
          type mw-types:power {
            range "-99..99";
          }
          units "dBm";
          mandatory true;
          description
            "Nominal maximum output power.";
          reference
            "ETSI EN 302 217-1";
        }
        leaf minimum-nominal-power {
          type mw-types:power {
            range "-99..99";
          }
          units "dBm";
          mandatory true;
          description
            "Minimum output power.

             By default, minimum output power is the same as the
             system capability minimum-power.";
        }
  }
[...]
    choice power-mode {
      mandatory true;
      description
        "A choice of RTPC or ATPC.";
      container rtpc {
        description
          "Remote Transmit Power Control (RTPC).";
        reference
          "ETSI EN 302 217-1";
        uses  nominal-power-limits;
      }
      container atpc {
        description
          "Automatic Transmitter Power Control (ATPC).";
        reference
          "ETSI EN 302 217-1";
        uses nominal-power-limits;
        [...]
      }
    }
[...]