sim: Move BitUnion overloading to show/parseParams

sim: Move BitUnion overloading to show/parseParams

This patch is moving template overloading for BitUnions into the
showParam, parseParams functions. Henceforth BitUnion types will use the
common param wrapper.
This patch implicitly implements (UN)SERIALIZE_CONTAINER for BitUnions.

Change-Id: I0e1faadb4afd4dc9de5dc5fca40041e349c9ba73
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/13636
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>