FixedIO__Modules with various kinds of probe ports (#4105)

* - add probe IOs to FixedIOModuleSpec
* getMatchedFields: don't recurse through probes.
* update scaladoc for FlatIO for probe types

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Co-authored-by: Will Dietz <will.dietz@sifive.com>
(cherry picked from commit 0553b694)

# Conflicts:
#	core/src/main/scala/chisel3/Data.scala