[top_earlgrey,ast] Add CSRs to control muxes on IOB[0:3] and wire them up

[andreaskurth]

PR #23201 will add muxes on IOB[0:3]. The selectors of those muxes are currently tied off (there are TODOs referencing this issue in the code). Those selectors instead need to be driven based on CSR bits in AST. So the following changes are required:

• Add CSR bits in AST (0: direct connection; 1: muxed to/from SPI dev/host; reset value: 0)
• Expose the values of those bits as outputs of AST.
• Connect these AST outputs to the mux selectors.

[andreaskurth]

@meisnere if you can implement the first two items, I can do the third one afterwards (as discussed yesterday)