mem-ruby: allow qualifiers in SLICC functions

mem-ruby: allow qualifiers in SLICC functions

All parameters in functions defined within SLICC are const& by default
(except for the implicit types, e.g. TBE). This allow us to specify
if we want to pass parameters as & or const&. Default behavior is
maintained.

A use case is to allow refactoring of common code in actions that
enqueue messages. Messages can be passed as a non-const ref. to
to functions with common initialization. E.g.:

void initRequestMsg(RequestMsg & out_msg) {
  // Common msg init code
}

action(sendRequest1, ...) {
  enqueue(...) {
    initRequestMsg(out_msg);
    // Request1 specific code
  }
}

action(sendRequest2, ...) {
  enqueue(...) {
    initRequestMsg(out_msg);
    // Request2 specific code
  }
}

Change-Id: Ic6a18169a661b3e36710b2a9f8a0e6bc5fce40f8
Signed-off-by: Tiago Mück <tiago.muck@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/31259
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>