Enable External Clock sources on the Xync

[sjkelly]

These are the changes required to enable external clock sources on the XTRX. Particularly the 19.2MHz clock sourced from the Xync carrier. There are some differences in (gain?) response and possibly noise.

External Clock (19.2 MHz):

Internal Clock (26 MHz):

[sjkelly]

For reference, to change the clock source:

    Device(first(Devices(;device_kwargs...))) do dev
        dev.clock_source = "external"
        ...

[staticfloat]

Why is SoapySDR bumped here?

[sjkelly]

We need the clock source support.

[staticfloat]

I see; merge when ready then!