Adding basic support for SX1255 Soapy driver

[Circu1tI0N3rd]

Addiing support for SX1255-based Raspberry Pi HATs as Soapy SDR devices.

This PR seeks to add support for Semtech SX1255 transceiver RX capability as a Raspberry Pi HAT using SoapySX driver for SoapySDR backend. It adds 4 different variants:

• M17 Project SX1255 HAT using 32 MHz TCXO as clock source;
• A variant of the M17 Project SX1255 HAT using 36 Mhz TCXO as clock source;
• A variant of the M17 Project SX1255 HAT using 36.864 Mhz TCXO as clock source;
• And OH2EAT SXceiver or a variant of the M17 Project SX1255 HAT using 38.4 Mhz TCXO as clock source.

The 4 variants reflect the difference in sample rates used due to the usage of different TCXOs.

[luarvique]

Please, consider adding these devices as a single source type, with the variant selection via a setting.

[Circu1tI0N3rd]

I'll review the class structure and try to merge... But I'm not sure if it's possible, since the sample rate selection alters significantly for each clock source used on the SX1255 HAT. For now, I'll remove the 36-MHz related sources, since the Soapy driver hasn't implemented methods to detect those clock frequencies yet, only 32 MHz and 38.4 MHz.

[luarvique]

Yep. That's the spirit! Will likely merge it for the next release, once 1.2.122 comes out.

[Circu1tI0N3rd]

I've merged all the redundant devices into one - sxceiver.py - with the following alterations:

• Adding clk_freq for variant reflection, specified as "required" in device settings;
• Replacing samp_rate with clk_prescaler for sample rate adaptiveness per clk_freq;
• Back-propagating now-missing samp_rate via a sample rate calculator function, handled at every possible samp_rate references within SdrSource

At the present, only two variants is supported: 32 MHz (M17 Project SX1255 HAT) and 38.4 MHz (OH2EAT SXceiver).