e320: Ensure GPSDO is cleanly reset upon radio power-on

[draeman-synoptic]

This patch performs an explicit reset on the GPSDO when the E320 MPM service is initialized upon boot. This ensures the GPSDO HW is always in a known power-on reset state when MPM starts up.

Description

It's worth noting that this PR doesn't address a specific known problem, but rather was a way to ensure the GPSDO HW is always brought into a known reinitialized state upon startup, using MPM functions that manage HW state. Previously initial pin levels were simply set when the FPGA is loaded, which may differ in timing/sequencing from MPM pin control logic.

I was on the fence about submitting as a PR since any benefit is unclear at best, but figured I'd defer to your judgement. I needed to ensure GPS was being initialized consistently across a cluster of E320 radios including upon soft reboot.

Which devices/areas does this affect?

MPM running on USRP E320.

Testing Done

Confirmed GPSDO operates as expected upon power-up. Confirmed GPSDO undergoes a HW reset if the USRP MPM service is restarted on the radio.

Checklist

• I have read the CONTRIBUTING document.
• My code follows the code style of this project. See CODING.md.
• I have updated the documentation accordingly.
• I have added tests to cover my changes, and all previous tests pass.
• I have checked all compat numbers if they need updating (FPGA compat,
  MPM compat, noc_shell, specific RFNoC block, ...)