rfnoc: Clarify usage of MTU vs. max payload size, remove DEFAULT_SPP

These two values where being mixed up in the code. To summarize:
- The MTU is the max CHDR packet size, including header & timestamp.
- The max payload is the total number of bytes regular payload plus
  metadata that can be fit into into a CHDR packet. It is strictly
  smaller than the MTU. For example, for 64-bit CHDR widths, if
  a timestamp is desired, the max payload is 16 bytes smaller than
  the MTU.

The other issue was that we were using a magic constant (DEFAULT_SPP)
which was causing conflicts with MTUs and max payloads.
This constant was harmful in multiple ways:
- The explanatory comment was incorrect (it stated it would cap packets
  to 1500 bytes, which it didn't)
- It imposed random, hardcoded values that interfered with an 'spp
  discovery', i.e., the ability to derive a good spp value from MTUs
- The current value capped packet sizes to 8000 bytes CHDR packets, even
  when we wanted to use bigger ones

This patch changes the following:
- noc_block_base now has improved docs for MTU, and additional APIs
  (get_max_payload_size(), get_chdr_hdr_len()) which return the
  current payload size given MTU and CHDR width, and the CHDR header
  length.
- The internally used graph nodes for TX and RX streamers also get
  equipped with the same new two API calls.
- The radio, siggen, and replay block all where doing different
  calculations for their spp/ipp values. Now, they all use the max
  payload value to calculate spp/ipp. Unit tests where adapted
  accordingly. Usage of DEFAULT_SPP was removed.
- The replay block used a hardcoded 16 bytes for header lengths, which
  was replaced by get_chdr_hdr_len()
- The TX and RX streamers where discarding the MTU value and using the
  max payload size as the MTU, which then propagated throughout the
  graph. Now, both values are stored and can be used where appropriate.

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