[opentitanlib] Expose transport clock frequencies through the ot_transport JSON5 files.

Description

When #30474 was merged, there was a need for the host to know the main clock frequency of the CW340 device that it was talking to (see here). Since we know this is only a CW340, it is just hard-coded for now. However:

1. There could be more FPGA targets in the future, which might have different clock speeds.
2. It makes changing the FPGA clocks more difficult (if desired).

In reality, the various clock frequencies that are already defined on the device side in the arch/device_{}.h files should also be available through the ot_transport JSON5 files, like the one here. This should then propagate through to the transport object, and allow users to query the frequency of a given device clock - just like they already can for pin strappings, UARTs, SPIs etc. This should be quite single, as this is just a single integer that needs to be passed through.

Start with sw/host/opentitanlib/src/io/fpga_backdoor.rs and the chipwhisperer/config/opentitan_cw340.json5 transport configuration, then compare the clock definitions in arch/device_{}.h with existing transport metadata such as pin strappings and UARTs. Done means the clock frequencies from each ot_transport JSON5 file propagate into the transport object and can be queried by users.