usdr_dm_create: Lack external clock settings

In the usdr_dm_create tool an extrernal clock can be selected but the frequency settings are missing.

Extclc path:
```
res = usdr_dme_set_string(dev, "/dm/sdr/refclk/path", refclkpath);
```

Freq set:
```
res = usdr_dme_set_uint(dev, "/dm/sdr/refclk/frequency", fref);
```

Freq get:
```
res = usdr_dme_get_uint(dev, "/dm/sdr/refclk/frequency", pfref);
```